Home / Input/Output Devices / Sensors and Transducers

Sensors and Transducers

Simple stand alone electronic circuits can be made to repeatedly flash a light or play a musical
note.

But in order for an electronic circuit or system to perform any useful task or function it needs to
be able to communicate with the “real world” whether this is by reading an input signal from an
“ON/OFF” switch or by activating some form of output device to illuminate a single light.
In other words, an Electronic System or circuit must be able to “do” something and Sensors
and Transducers are the perfect components for doing this.
Related Products: Ambient Light Sensor | Temperature and Humidity Sensors
The word “Transducer” is the collective term used for both Sensors which can be used to sense a
wide range of different energy forms such as movement, electrical signals, radiant energy,
thermal or magnetic energy etc, and Actuators which can be used to switch voltages or currents.
There are many different types of sensors and transducers, both analogue and digital and input
and output available to choose from. The type of input or output transducer being used, really
depends upon the type of signal or process being “Sensed” or “Controlled” but we can define a
sensor and transducers as devices that converts one physical quantity into another.
Devices which perform an “Input” function are commonly called Sensors because they “sense” a
physical change in some characteristic that changes in response to some excitation, for example
heat or force and covert that into an electrical signal. Devices which perform an “Output”
function are generally called Actuators and are used to control some external device, for
example movement or sound.
Electrical Transducers are used to convert energy of one kind into energy of another kind, so
for example, a microphone (input device) converts sound waves into electrical signals for the
amplifier to amplify (a process), and a loudspeaker (output device) converts these electrical
signals back into sound waves and an example of this type of simple Input/Output (I/O) system is
given below.

Simple Input/Output System using Sound Transducers

There are many different types of sensors and transducers available in the marketplace, and the
choice of which one to use really depends upon the quantity being measured or controlled, with
the more common types given in the table below.

Common Sensors and Transducers

Quantity being Input Device Output Device

Measured (Sensor) (Actuator)

Light Dependant Resistor (LDR)

Lights & Lamps
Photodiode
Light Level LED’s & Displays
Photo-transistor
Fibre Optics
Solar Cell

Thermocouple
Thermistor Heater
Temperature
Thermostat Fan
Resistive Temperature Detectors

Strain Gauge Lifts & Jacks

Force/Pressure Pressure Switch Electromagnet
Load Cells Vibration

Potentiometer
Motor
Encoders
Position Solenoid
Reflective/Slotted Opto-switch
Panel Meters
LVDT
 Tacho-generator AC and DC Motors
Speed Reflective/Slotted Opto-coupler Stepper Motor
Doppler Effect Sensors Brake

Bell
Carbon Microphone
Sound Buzzer
Piezo-electric Crystal
Loudspeaker

Input type transducers or sensors, produce a voltage or signal output response which is
proportional to the change in the quantity that they are measuring (the stimulus). The type or
amount of the output signal depends upon the type of sensor being used. But generally, all types
of sensors can be classed as two kinds, either Passive Sensors or Active Sensors.
Generally, active sensors require an external power supply to operate, called an excitation
signal which is used by the sensor to produce the output signal. Active sensors are self-
generating devices because their own properties change in response to an external effect
producing for example, an output voltage of 1 to 10v DC or an output current such as 4 to 20mA
DC. Active sensors can also produce signal amplification.
A good example of an active sensor is an LVDT sensor or a strain gauge. Strain gauges are
pressure-sensitive resistive bridge networks that are external biased (excitation signal) in such a
way as to produce an output voltage in proportion to the amount of force and/or strain being
applied to the sensor.
Unlike an active sensor, a passive sensor does not need any additional power source or excitation
voltage. Instead a passive sensor generates an output signal in response to some external
stimulus. For example, a thermocouple which generates its own voltage output when exposed to
heat. Then passive sensors are direct sensors which change their physical properties, such as
resistance, capacitance or inductance etc.
But as well as analogue sensors, Digital Sensors produce a discrete output representing a binary
number or digit such as a logic level “0” or a logic level “1”.

Analogue and Digital Sensors

Analogue Sensors
Analogue Sensors produce a continuous output signal or voltage which is generally proportional
to the quantity being measured. Physical quantities such as Temperature, Speed, Pressure,
Displacement, Strain etc are all analogue quantities as they tend to be continuous in nature. For
example, the temperature of a liquid can be measured using a thermometer or thermocouple
which continuously responds to temperature changes as the liquid is heated up or cooled down.
Thermocouple used to produce an Analogue Signal

Analogue sensors tend to produce output signals that are changing smoothly and continuously
over time. These signals tend to be very small in value from a few mico-volts (uV) to several
milli-volts (mV), so some form of amplification is required.
Then circuits which measure analogue signals usually have a slow response and/or low accuracy.
Also analogue signals can be easily converted into digital type signals for use in micro-controller
systems by the use of analogue-to-digital converters, or ADC’s.

Digital Sensors
As its name implies, Digital Sensors produce a discrete digital output signals or voltages that are
a digital representation of the quantity being measured. Digital sensors produce a Binary output
signal in the form of a logic “1” or a logic “0”, (“ON” or “OFF”). This means then that a digital
signal only produces discrete (non-continuous) values which may be outputted as a single “bit”,
(serial transmission) or by combining the bits to produce a single “byte” output (parallel
transmission).

Light Sensor used to produce an Digital Signal

In our simple example above, the speed of the rotating shaft is measured by using a digital
LED/Opto-detector sensor. The disc which is fixed to a rotating shaft (for example, from a motor
or robot wheels), has a number of transparent slots within its design. As the disc rotates with the
speed of the shaft, each slot passes by the sensor in turn producing an output pulse representing a
logic “1” or logic “0” level.
These pulses are sent to a register of counter and finally to an output display to show the speed or
revolutions of the shaft. By increasing the number of slots or “windows” within the disc more
output pulses can be produced for each revolution of the shaft. The advantage of this is that a
greater resolution and accuracy is achieved as fractions of a revolution can be detected. Then this
type of sensor arrangement could also be used for positional control with one of the discs slots
representing a reference position.
Compared to analogue signals, digital signals or quantities have very high accuracies and can be
both measured and “sampled” at a very high clock speed. The accuracy of the digital signal is
proportional to the number of bits used to represent the measured quantity. For example, using a
processor of 8 bits, will produce an accuracy of 0.390% (1 part in 256). While using a processor
of 16 bits gives an accuracy of 0.0015%, (1 part in 65,536) or 260 times more accurate. This
accuracy can be maintained as digital quantities are manipulated and processed very rapidly,
millions of times faster than analogue signals.
In most cases, sensors and more specifically analogue sensors generally require an external
power supply and some form of additional amplification or filtering of the signal in order to
produce a suitable electrical signal which is capable of being measured or used. One very good
way of achieving both amplification and filtering within a single circuit is to use Operational
Amplifiers as seen before.

Signal Conditioning of Sensors

As we saw in the Operational Amplifier tutorial, op-amps can be used to provide amplification
of signals when connected in either inverting or non-inverting configurations.
The very small analogue signal voltages produced by a sensor such as a few milli-volts or even
pico-volts can be amplified many times over by a simple op-amp circuit to produce a much
larger voltage signal of say 5v or 5mA that can then be used as an input signal to a
microprocessor or analogue-to-digital based system.
Therefore, to provide any useful signal a sensors output signal has to be amplified with an
amplifier that has a voltage gain up to 10,000 and a current gain up to 1,000,000 with the
amplification of the signal being linear with the output signal being an exact reproduction of the
input, just changed in amplitude.
Then amplification is part of signal conditioning. So when using analogue sensors, generally
some form of amplification (Gain), impedance matching, isolation between the input and output
or perhaps filtering (frequency selection) may be required before the signal can be used and this
is conveniently performed by Operational Amplifiers.
Also, when measuring very small physical changes the output signal of a sensor can become
“contaminated” with unwanted signals or voltages that prevent the actual signal required from
being measured correctly. These unwanted signals are called “Noise“. This Noise or Interference
can be either greatly reduced or even eliminated by using signal conditioning or filtering
techniques as we discussed in the Active Filtertutorial.
By using either a Low Pass, or a High Pass or even Band Pass filter the “bandwidth” of the
noise can be reduced to leave just the output signal required. For example, many types of inputs
from switches, keyboards or manual controls are not capable of changing state rapidly and so
low-pass filter can be used. When the interference is at a particular frequency, for example mains
frequency, narrow band reject or Notch filters can be used to produce frequency selective filters.

Typical Op-amp Filters

Were some random noise still remains after filtering it may be necessary to take several samples
and then average them to give the final value so increasing the signal-to-noise ratio. Either way,
both amplification and filtering play an important role in interfacing both sensors and transducers
to microprocessor and electronics based systems in “real world” conditions.
In the next tutorial about Sensors, we will look at Positional Sensors which measure the position
and/or displacement of physical objects meaning the movement from one position to another for
a specific distance or angle.
Previous
Inductive Reactance

Next
Position Sensors

Other Tutorials in Input/Output Devices

 Summary of Transducers
 Sound Transducers
 DC Motors
 Linear Solenoid Actuator
 Electrical Relay
 Light Sensors
 Temperature Sensors
 Position Sensors
 Sensors and Transducers
Home / Input/Output Devices / Position Sensors
Position Sensors
In this tutorial we will look at a variety of devices which are classed as Input Devices and are
therefore called “Sensors” and in particular those sensors which are Positional in nature.

As their name implies, Position Sensors detect the position of something which means that they
are referenced either to or from some fixed point or position. These types of sensors provide a
“positional” feedback.
One method of determining a position, is to use either “distance”, which could be the distance
between two points such as the distance travelled or moved away from some fixed point, or by
“rotation” (angular movement). For example, the rotation of a robots wheel to determine its
distance travelled along the ground. Either way, Position Sensors can detect the movement of an
object in a straight line using Linear Sensors or by its angular movement using Rotational
Sensors.
Related Products: Angular and Linear Position Sensor

The Potentiometer
The most commonly used of all the “Position Sensors”, is the potentiometer because it is an
inexpensive and easy to use position sensor. It has a wiper contact linked to a mechanical shaft
that can be either angular (rotational) or linear (slider type) in its movement, and which causes
the resistance value between the wiper/slider and the two end connections to change giving an
electrical signal output that has a proportional relationship between the actual wiper position on
the resistive track and its resistance value. In other words, resistance is proportional to position.

Potentiometer
Potentiometers come in a wide range of designs and sizes such as the commonly available round
rotational type or the longer and flat linear slider types. When used as a position sensor the
moveable object is connected directly to the rotational shaft or slider of the potentiometer.
A DC reference voltage is applied across the two outer fixed connections forming the resistive
element. The output voltage signal is taken from the wiper terminal of the sliding contact as
shown below.
This configuration produces a potential or voltage divider type circuit output which is
proportional to the shaft position. Then for example, if you apply a voltage of say 10v across the
resistive element of the potentiometer the maximum output voltage would be equal to the supply
voltage at 10 volts, with the minimum output voltage equal to 0 volts. Then the potentiometer
wiper will vary the output signal from 0 to 10 volts, with 5 volts indicating that the wiper or
slider is at its half-way or centre position.

Potentiometer Construction

The output signal (Vout) from the potentiometer is taken from the centre wiper connection as it
moves along the resistive track, and is proportional to the angular position of the shaft.

Example of a simple Positional Sensing Circuit

While resistive potentiometer position sensors have many advantages: low cost, low tech, easy to
use etc, as a position sensor they also have many disadvantages: wear due to moving parts, low
accuracy, low repeatability, and limited frequency response.
But there is one main disadvantage of using the potentiometer as a positional sensor. The range
of movement of its wiper or slider (and hence the output signal obtained) is limited to the
physical size of the potentiometer being used.
For example a single turn rotational potentiometer generally only has a fixed mechanical rotation
of between 0o and about 240 to 330o maximum. However, multi-turn pots of up to 3600o (10 x
360o) of mechanical rotation are also available.
Most types of potentiometers use carbon film for their resistive track, but these types are
electrically noisy (the crackle on a radio volume control), and also have a short mechanical life.
Wire-wound pots also known as rheostats, in the form of either a straight wire or wound coil
resistive wire can also be used, but wire wound pots suffer from resolution problems as their
wiper jumps from one wire segment to the next producing a logarithmic (LOG) output resulting
in errors in the output signal. These too suffer from electrical noise.
For high precision low noise applications conductive plastic resistance element type polymer
film or cermet type potentiometers are now available. These pots have a smooth low friction
electrically linear (LIN) resistive track giving them a low noise, long life and excellent resolution
and are available as both multi-turn and single turn devices. Typical applications for this type of
high accuracy position sensor is in computer game joysticks, steering wheels, industrial and
robot applications.

Inductive Position Sensors

Linear Variable Differential Transformer

One type of positional sensor that does not suffer from mechanical wear problems is the “Linear
Variable Differential Transformer” or LVDT for short. This is an inductive type position sensor
which works on the same principle as the AC transformer that is used to measure movement. It is
a very accurate device for measuring linear displacement and whose output is proportional to the
position of its moveable core.
It basically consists of three coils wound on a hollow tube former, one forming the primary coil
and the other two coils forming identical secondaries connected electrically together in series but
180o out of phase either side of the primary coil.
A moveable soft iron ferromagnetic core (sometimes called an “armature”) which is connected to
the object being measured, slides or moves up and down inside the tubular body of the LVDT.
A small AC reference voltage called the “excitation signal” (2 – 20V rms, 2 – 20kHz) is applied
to the primary winding which in turn induces an EMF signal into the two adjacent secondary
windings (transformer principles).
If the soft iron magnetic core armature is exactly in the centre of the tube and the windings, “null
position”, the two induced emf’s in the two secondary windings cancel each other out as they are
180o out of phase, so the resultant output voltage is zero. As the core is displaced slightly to one
side or the other from this null or zero position, the induced voltage in one of the secondaries
will be become greater than that of the other secondary and an output will be produced.
The polarity of the output signal depends upon the direction and displacement of the moving
core. The greater the movement of the soft iron core from its central null position the greater will
be the resulting output signal. The result is a differential voltage output which varies linearly
with the cores position. Therefore, the output signal from this type of position sensor has both an
amplitude that is a linear function of the cores displacement and a polarity that indicates
direction of movement.
The phase of the output signal can be compared to the primary coil excitation phase enabling
suitable electronic circuits such as the AD592 LVDT Sensor Amplifier to know which half of the
coil the magnetic core is in and thereby know the direction of travel.

The Linear Variable Differential Transformer

When the armature is moved from one end to the other through the centre position the output
voltages changes from maximum to zero and back to maximum again but in the process changes
its phase angle by 180 deg’s. This enables the LVDT to produce an output AC signal whose
magnitude represents the amount of movement from the centre position and whose phase angle
represents the direction of movement of the core.
A typical application of a linear variable differential transformer (LVDT) sensor would be as a
pressure transducer, were the pressure being measured pushes against a diaphragm to produce a
force. The force is then converted into a readable voltage signal by the sensor.
Advantages of the linear variable differential transformer, or LVDT compared to a resistive
potentiometer are that its linearity, that is its voltage output to displacement is excellent, very
good accuracy, good resolution, high sensitivity as well as frictionless operation. They are also
sealed for use in hostile environments.

Inductive Proximity Sensors.

Another type of inductive position sensor in common use is the Inductive Proximity
Sensor also called an Eddy current sensor. While they do not actually measure displacement or
angular rotation they are mainly used to detect the presence of an object in front of them or
within a close proximity, hence their name “proximity sensor“.
Proximity sensors, are non-contact position sensors that use a magnetic field for detection with
the simplest magnetic sensor being the reed switch. In an inductive sensor, a coil is wound
around an iron core within an electromagnetic field to form an inductive loop.
When a ferromagnetic material is placed within the eddy current field generated around the
inductive sensor, such as a ferromagnetic metal plate or metal screw, the inductance of the coil
changes significantly. The proximity sensors detection circuit detects this change producing an
output voltage. Therefore, inductive proximity sensors operate under the electrical principle
of Faraday’s Law of inductance.

Inductive Proximity Sensors

An inductive proximity sensor has four main components; The oscillator which produces the
electromagnetic field, the coil which generates the magnetic field, the detection circuit which
detects any change in the field when an object enters it and the output circuit which produces the
output signal, either with normally closed (NC) or normally open (NO) contacts.
Inductive proximity sensors allow for the detection of metallic objects in front of the sensor head
without any physical contact of the object itself being detected. This makes them ideal for use in
dirty or wet environments. The “sensing” range of proximity sensors is very small, typically
0.1mm to 12mm.

Proximity Sensor
As well as industrial applications, inductive proximity sensors are also commonly used to control
the flow of traffic by changing of traffic lights at junctions and cross roads. Rectangular
inductive loops of wire are buried into the tarmac road surface.
When a car or other road vehicle passes over this inductive loop, the metallic body of the vehicle
changes the loops inductance and activates the sensor thereby alerting the traffic lights controller
that there is a vehicle waiting.
One main disadvantage of these types of position sensors is that they are “Omni-directional”,
that is they will sense a metallic object either above, below or to the side of it. Also, they do not
detect non-metallic objects although Capacitive Proximity Sensorsand Ultrasonic Proximity
Sensors are available. Other commonly available magnetic positional sensors include: reed
switches, Hall Effect Sensors and variable reluctance sensors.

Rotary Encoders
Rotary Encoders are another type of position sensor which resemble potentiometers mentioned
earlier but are non-contact optical devices used for converting the angular position of a rotating
shaft into an analogue or digital data code. In other words, they convert mechanical movement
into an electrical signal (preferably digital).
All optical encoders work on the same basic principle. Light from an LED or infra-red
light source is passed through a rotating high-resolution encoded disk that contains the required
code patterns, either binary, grey code or BCD. Photo detectors scan the disk as it rotates and an
electronic circuit processes the information into a digital form as a stream of binary output pulses
that are fed to counters or controllers which determine the actual angular position of the shaft.
There are two basic types of rotary optical encoders, Incremental Encoders and Absolute
Position Encoders.

Incremental Encoder

Encoder Disk
Incremental Encoders, also known as quadrature encoders or relative rotary encoder, are the
simplest of the two position sensors. Their output is a series of square wave pulses generated by
a photocell arrangement as the coded disk, with evenly spaced transparent and dark lines called
segments on its surface, moves or rotates past the light source. The encoder produces a stream of
square wave pulses which, when counted, indicates the angular position of the rotating shaft.
Incremental encoders have two separate outputs called “quadrature outputs”. These two outputs
are displaced at 90o out of phase from each other with the direction of rotation of the shaft being
determined from the output sequence.
The number of transparent and dark segments or slots on the disk determines the resolution of
the device and increasing the number of lines in the pattern increases the resolution per degree of
rotation. Typical encoded discs have a resolution of up to 256 pulses or 8-bits per rotation.
The simplest incremental encoder is called a tachometer. It has one single square wave output
and is often used in unidirectional applications where basic position or speed information only is
required. The “Quadrature” or “Sine wave” encoder is the more common and has two output
square waves commonly called channel A and channel B. This device uses two photo detectors,
slightly offset from each other by 90o thereby producing two separate sine and cosine output
signals.

Simple Incremental Encoder

By using the Arc Tangent mathematical function the angle of the shaft in radians can be
calculated. Generally, the optical disk used in rotary position encoders is circular, then the
resolution of the output will be given as: θ = 360/n, where n equals the number of segments on
coded disk.
Then for example, the number of segments required to give an incremental encoder a resolution
of 1o will be: 1o = 360/n, therefore, n = 360 windows, etc. Also the direction of rotation is
determined by noting which channel produces an output first, either channel A or channel B
giving two directions of rotation, A leads B or B leads A. This arrangement is shown below.

Incremental Encoder Output

One main disadvantage of incremental encoders when used as a position sensor, is that they
require external counters to determine the absolute angle of the shaft within a given rotation. If
the power is momentarily shut off, or if the encoder misses a pulse due to noise or a dirty disc,
the resulting angular information will produce an error. One way of overcoming this
disadvantage is to use absolute position encoders.

Absolute Position Encoder

Absolute Position Encoders are more complex than quadrature encoders. They provide a
unique output code for every single position of rotation indicating both position and direction.
Their coded disk consists of multiple concentric “tracks” of light and dark segments. Each track
is independent with its own photo detector to simultaneously read a unique coded position value
for each angle of movement. The number of tracks on the disk corresponds to the binary “bit”-
resolution of the encoder so a 12-bit absolute encoder would have 12 tracks and the same coded
value only appears once per revolution.

4-bit Binary Coded Disc

One main advantage of an absolute encoder is its non-volatile memory which retains the exact
position of the encoder without the need to return to a “home” position if the power fails. Most
rotary encoders are defined as “single-turn” devices, but absolute multi-turn devices are
available, which obtain feedback over several revolutions by adding extra code disks.
Typical application of absolute position encoders are in computer hard drives and CD/DVD
drives were the absolute position of the drives read/write heads are monitored or in
printers/plotters to accurately position the printing heads over the paper.
In this tutorial about Position Sensors, we have looked at several examples of sensors that can
be used to measure the position or presence of objects. In the next tutorial we will look at sensors
that are used to measure temperature such as thermistors, thermostats and thermocouples, and as
such are known commonly as Temperature Sensors.
Previous
Sensors and Transducers

Next
Temperature Sensors

Other Tutorials in Input/Output Devices

 Summary of Transducers
 Sound Transducers
 DC Motors
 Linear Solenoid Actuator
 Electrical Relay
 Light Sensors
 Temperature Sensors
 Position Sensors
 Sensors and Transducers

Advertisement

Home / Input/Output Devices / Temperature Sensors

Temperature Sensors
The most commonly used type of all the sensors are those which detect Temperature or heat.

These types of temperature sensor vary from simple ON/OFF thermostatic devices which control
a domestic hot water heating system to highly sensitive semiconductor types that can control
complex process control furnace plants.
We remember from our school science classes that the movement of molecules and atoms
produces heat (kinetic energy) and the greater the movement, the more heat that is
generated. Temperature Sensors measure the amount of heat energy or even coldness that is
generated by an object or system, allowing us to “sense” or detect any physical change to that
temperature producing either an analogue or digital output.
There are many different types of Temperature Sensor available and all have different
characteristics depending upon their actual application. A temperature sensor consists of two
basic physical types:
 Contact Temperature Sensor Types – These types of temperature sensor are required to
be in physical contact with the object being sensed and use conduction to monitor changes in
temperature. They can be used to detect solids, liquids or gases over a wide range of temperatures.
 Non-contact Temperature Sensor Types – These types of temperature sensor use
convection and radiation to monitor changes in temperature. They can be used to detect liquids
and gases that emit radiant energy as heat rises and cold settles to the bottom in convection
currents or detect the radiant energy being transmitted from an object in the form of infra-red
radiation (the sun).
The two basic types of contact or even non-contact temperature sensors can also be sub-divided
into
the following three groups of sensors, Electro-mechanical, Resistive and Electronic and all three
types are discussed below.

The Thermostat
The Thermostat is a contact type electro-mechanical temperature sensor or switch, that basically
consists of two different metals such as nickel, copper, tungsten or aluminium etc, that are
bonded together to form a Bi-metallic strip. The different linear expansion rates of the two
dissimilar metals produces a mechanical bending movement when the strip is subjected to heat.
The bi-metallic strip can be used itself as an electrical switch or as a mechanical way of
operating an electrical switch in thermostatic controls and are used extensively to control hot
water heating elements in boilers, furnaces, hot water storage tanks as well as in vehicle radiator
cooling systems.

The Bi-metallic Thermostat

The thermostat consists of two thermally different metals stuck together back to back. When it is
cold the contacts are closed and current passes through the thermostat. When it gets hot, one
metal expands more than the other and the bonded bi-metallic strip bends up (or down) opening
the contacts preventing the current from flowing.

On/Off Thermostat

There are two main types of bi-metallic strips based mainly upon their movement when
subjected to temperature changes. There are the “snap-action” types that produce an
instantaneous “ON/OFF” or “OFF/ON” type action on the electrical contacts at a set temperature
point, and the slower “creep-action” types that gradually change their position as the temperature
changes.
Snap-action type thermostats are commonly used in our homes for controlling the temperature
set point of ovens, irons, immersion hot water tanks and they can also be found on walls to
control the domestic heating system.
Creeper types generally consist of a bi-metallic coil or spiral that slowly unwinds or coils-up as
the temperature changes. Generally, creeper type bi-metallic strips are more sensitive to
temperature changes than the standard snap ON/OFF types as the strip is longer and thinner
making them ideal for use in temperature gauges and dials etc.
Although very cheap and are available over a wide operating range, one main disadvantage of
the standard snap-action type thermostats when used as a temperature sensor, is that they have a
large hysteresis range from when the electrical contacts open until when they close again. For
example, it may be set to 20oC but may not open until 22oC or close again until 18oC.
So the range of temperature swing can be quite high. Commercially available bi-metallic
thermostats for home use do have temperature adjustment screws that allow for a more precise
desired temperature set-point and hysteresis level to be pre-set.

The Thermistor
The Thermistor is another type of temperature sensor, whose name is a combination of the
words THERM-ally sensitive res-ISTOR. A thermistor is a special type of resistor which
changes its physical resistance when exposed to changes in temperature.

fThermistor

Thermistors are generally made from ceramic materials such as oxides of nickel, manganese or
cobalt coated in glass which makes them easily damaged. Their main advantage over snap-action
types is their speed of response to any changes in temperature, accuracy and repeatability.
Most types of thermistor’s have a Negative Temperature Coefficient of resistance or (NTC), that
is their resistance value goes DOWN with an increase in the temperature, and of course there are
some which have a Positive Temperature Coefficient, (PTC), in that their resistance value goes
UP with an increase in temperature.
Thermistors are constructed from a ceramic type semiconductor material using metal oxide
technology such as manganese, cobalt and nickel, etc. The semiconductor material is generally
formed into small pressed discs or balls which are hermetically sealed to give a relatively fast
response to any changes in temperature.
Thermistors are rated by their resistive value at room temperature (usually at 25oC), their time
constant (the time to react to the temperature change) and their power rating with respect to the
current flowing through them. Like resistors, thermistors are available with resistance values at
room temperature from 10’s of MΩ down to just a few Ohms, but for sensing purposes those
types with values in the kilo-ohms are generally used.
Thermistors are passive resistive devices which means we need to pass a current through it to
produce a measurable voltage output. Then thermistors are generally connected in series with a
suitable biasing resistor to form a potential divider network and the choice of resistor gives a
voltage output at some pre-determined temperature point or value for example:

Temperature Sensors Example No1

The following thermistor has a resistance value of 10KΩ at 25oC and a resistance value of 100Ω
at 100oC. Calculate the voltage drop across the thermistor and hence its output voltage (Vout) for
both temperatures when connected in series with a 1kΩ resistor across a 12v power supply.

At 25oC

At 100oC

By changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or preset, a
voltage output can be obtained at a predetermined temperature set point for example, 5v output at
60oC and by varying the potentiometer a particular output voltage level can be obtained over a
wider temperature range.
It needs to be noted however, that thermistor’s are non-linear devices and their standard
resistance values at room temperature is different between different thermistor’s, which is due
mainly to the semiconductor materials they are made from. The Thermistor, have an
exponential change with temperature and therefore have a Beta temperature constant ( β ) which
can be used to calculate its resistance for any given temperature point.
However, when used with a series resistor such as in a voltage divider network or Whetstone
Bridge type arrangement, the current obtained in response to a voltage applied to the
divider/bridge network is linear with temperature. Then, the output voltage across the resistor
becomes linear with temperature.

Resistive Temperature Detectors (RTD).

Another type of electrical resistance temperature sensor is the Resistance Temperature
Detector or RTD. RTD’s are precision temperature sensors made from high-purity conducting
metals such as platinum, copper or nickel wound into a coil and whose electrical resistance
changes as a function of temperature, similar to that of the thermistor. Also available are thin-
film RTD’s. These devices have a thin film of platinum paste is deposited onto a white ceramic
substrate.

A Resistive RTD

Resistive temperature detectors have positive temperature coefficients (PTC) but unlike the
thermistor their output is extremely linear producing very accurate measurements of temperature.
However, they have very poor thermal sensitivity, that is a change in temperature only produces
a very small output change for example, 1Ω/oC.
The more common types of RTD’s are made from platinum and are called Platinum Resistance
Thermometer or PRT‘s with the most commonly available of them all the Pt100 sensor, which
has a standard resistance value of 100Ω at 0oC. The downside is that Platinum is expensive and
one of the main disadvantages of this type of device is its cost.
Like the thermistor, RTD’s are passive resistive devices and by passing a constant current
through the temperature sensor it is possible to obtain an output voltage that increases linearly
with temperature. A typical RTD has a base resistance of about 100Ω at 0oC, increasing to about
140Ω at 100oC with an operating temperature range of between -200 to +600oC.
Because the RTD is a resistive device, we need to pass a current through them and monitor the
resulting voltage. However, any variation in resistance due to self heat of the resistive wires as
the current flows through it, I2R , (Ohms Law) causes an error in the readings. To avoid this, the
RTD is usually connected into a Whetstone Bridge network which has additional connecting
wires for lead-compensation and/or connection to a constant current source.

The Thermocouple
The Thermocouple is by far the most commonly used type of all the temperature sensor types.
Thermocouples are popular due to its simplicity, ease of use and their speed of response to
changes in temperature, due mainly to their small size. Thermocouples also have the widest
temperature range of all the temperature sensors from below -200oC to well over 2000oC.
Thermocouples are thermoelectric sensors that basically consists of two junctions of dissimilar
metals, such as copper and constantan that are welded or crimped together. One junction is kept
at a constant temperature called the reference (Cold) junction, while the other the measuring
(Hot) junction. When the two junctions are at different temperatures, a voltage is developed
across the junction which is used to measure the temperature sensor as shown below.

Thermocouple Construction

The operating principal of a thermocouple is very simple and basic. When fused together the
junction of the two dissimilar metals such as copper and constantan produces a “thermo-electric”
effect which gives a constant potential difference of only a few millivolts (mV) between them.
The voltage difference between the two junctions is called the “Seebeck effect” as a temperature
gradient is generated along the conducting wires producing an emf. Then the output voltage from
a thermocouple is a function of the temperature changes.
If both the junctions are at the same temperature the potential difference across the two junctions
is zero in other words, no voltage output as V1 = V2. However, when the junctions are connected
within a circuit and are both at different temperatures a voltage output will be detected relative to
the difference in temperature between the two junctions, V1 – V2. This difference in voltage will
increase with temperature until the junctions peak voltage level is reached and this is determined
by the characteristics of the two dissimilar metals used.
Thermocouples can be made from a variety of different materials enabling extreme temperatures
of
between -200oC to over +2000oC to be measured. With such a large choice of materials and
temperature range, internationally recognised standards have been developed complete with
thermocouple colour codes to allow the user to choose the correct thermocouple sensor for a
particular application. The British colour code for standard thermocouples is given below.

Thermocouple Colour Codes

Thermocouple Sensor Colour Codes

Extension and Compensating Leads
Code British
Conductors (+/-) Sensitivity
Type BS 1843:1952

Nickel Chromium /
E -200 to 900oC
Constantan

J Iron / Constantan 0 to 750oC

Nickel Chromium /
K -200 to 1250oC
Nickel Aluminium

N Nicrosil / Nisil 0 to 1250oC

T Copper / Constantan -200 to 350oC

Copper / Copper Nickel

U Compensating for 0 to 1450oC
“S” and “R”
The three most common thermocouple materials used above for general temperature
measurement are Iron-Constantan (Type J), Copper-Constantan (Type T), and Nickel-
Chromium (Type K). The output voltage from a thermocouple is very small, only a few
millivolts (mV) for a 10oC change in temperature difference and because of this small voltage
output some form of amplification is generally required.

Thermocouple Amplification

The type of amplifier, either discrete or in the form of an Operational Amplifier needs to be
carefully selected, because good drift stability is required to prevent recalibration of the
thermocouple at frequent intervals. This makes the chopper and instrumentation type of amplifier
preferable for most temperature sensing applications.
Other Temperature Sensor Types not mentioned here include, Semiconductor Junction
Sensors, Infra-red and Thermal Radiation Sensors, Medical type Thermometers, Indicators and
Colour Changing Inks or Dyes.
In this tutorial about “Temperature Sensor Types”, we have looked at several examples of
sensors that can be used to measure changes in temperature. In the next tutorial we will look at
sensors that are used to measure light quantity, such as Photodiodes, Phototransistors,
Photovoltaic Cells and the Light Dependant Resistor.

Previous
Position Sensors
Next
Light Sensors

Home / Input/Output Devices / Light Sensors

Light Sensors
A Light Sensor generates an output signal indicating the intensity of light by measuring the
radiant energy that exists in a very narrow range of frequencies basically called “light”, and
which ranges in frequency from “Infra-red” to “Visible” up to “Ultraviolet” light spectrum.

The light sensor is a passive devices that convert this “light energy” whether visible or in the
infra-red parts of the spectrum into an electrical signal output. Light sensors are more commonly
known as “Photoelectric Devices” or “Photo Sensors” because the convert light energy (photons)
into electricity (electrons).
Photoelectric devices can be grouped into two main categories, those which generate electricity
when illuminated, such as Photo-voltaics or Photo-emissives etc, and those which change their
electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the
following classification of devices.
 • Photo-emissive Cells – These are photodevices which release free electrons from a light
sensitive material such as caesium when struck by a photon of sufficient energy. The amount of
energy the photons have depends on the frequency of the light and the higher the frequency, the
more energy the photons have converting light energy into electrical energy.
 • Photo-conductive Cells – These photodevices vary their electrical resistance when
subjected to light. Photoconductivity results from light hitting a semiconductor material which
controls the current flow through it. Thus, more light increase the current for a given applied
voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.
 • Photo-voltaic Cells – These photodevices generate an emf in proportion to the radiant light
energy received and is similar in effect to photoconductivity. Light energy falls on to two
semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most
common photovoltaic material is Selenium used in solar cells.
 • Photo-junction Devices – These photodevices are mainly true semiconductor devices such
as the photodiode or phototransistor which use light to control the flow of electrons and holes
across their PN-junction. Photojunction devices are specifically designed for detector application
and light penetration with their spectral response tuned to the wavelength of incident light.
Related Products: Ambient Light Sensor | Photoelectric Sensor

The Photoconductive Cell

A Photoconductive light sensor does not produce electricity but simply changes its physical
properties when subjected to light energy. The most common type of photoconductive device is
the Photoresistor which changes its electrical resistance in response to changes in the light
intensity.
Photoresistors are Semiconductor devices that use light energy to control the flow of electrons,
and hence the current flowing through them. The commonly used Photoconductive Cell is called
the Light Dependent Resistor or LDR.

The Light Dependent Resistor

Typical LDR

As its name implies, the Light Dependent Resistor (LDR) is made from a piece of exposed
semiconductor material such as cadmium sulphide that changes its electrical resistance from
several thousand Ohms in the dark to only a few hundred Ohms when light falls upon it by
creating hole-electron pairs in the material.
The net effect is an improvement in its conductivity with a decrease in resistance for an increase
in illumination. Also, photoresistive cells have a long response time requiring many seconds to
respond to a change in the light intensity.
Materials used as the semiconductor substrate include, lead sulphide (PbS), lead selenide (PbSe),
indium antimonide (InSb) which detect light in the infra-red range with the most commonly used
of all photoresistive light sensors being Cadmium Sulphide (Cds).
Cadmium sulphide is used in the manufacture of photoconductive cells because its spectral
response curve closely matches that of the human eye and can even be controlled using a simple
torch as a light source. Typically then, it has a peak sensitivity wavelength (λp) of about 560nm
to 600nm in the visible spectral range.

The Light Dependent Resistor Cell

The most commonly used photoresistive light sensor is the ORP12 Cadmium Sulphide
photoconductive cell. This light dependent resistor has a spectral response of about 610nm in the
yellow to orange region of light. The resistance of the cell when unilluminated (dark resistance)
is very high at about 10MΩ’s which falls to about 100Ω’s when fully illuminated (lit resistance).
To increase the dark resistance and therefore reduce the dark current, the resistive path forms a
zigzag pattern across the ceramic substrate. The CdS photocell is a very low cost device often
used in auto dimming, darkness or twilight detection for turning the street lights “ON” and
“OFF”, and for photographic exposure meter type applications.

Connecting a light dependant resistor in series with a standard resistor like this across a single
DC supply voltage has one major advantage, a different voltage will appear at their junction for
different levels of light.
The amount of voltage drop across series resistor, R2is determined by the resistive value of the
light dependant resistor, RLDR. This ability to generate different voltages produces a very handy
circuit called a “Potential Divider” or Voltage Divider Network.
As we know, the current through a series circuit is common and as the LDR changes its resistive
value due to the light intensity, the voltage present at VOUT will be determined by the voltage
divider formula. An LDR’s resistance, RLDR can vary from about 100Ω’s in the sun light, to over
10MΩ’s in absolute darkness with this variation of resistance being converted into a voltage
variation at VOUT as shown.
One simple use of a Light Dependent Resistor, is as a light sensitive switch as shown below.

LDR Switch

This basic light sensor circuit is of a relay output light activated switch. A potential divider
circuit is formed between the photoresistor, LDR and the resistor R1. When no light is present ie
in darkness, the resistance of the LDR is very high in the Megaohms (MΩ’s) range so zero base
bias is applied to the transistor TR1 and the relay is de-energised or “OFF”.
As the light level increases the resistance of the LDR starts to decrease causing the base bias
voltage at V1 to rise. At some point determined by the potential divider network formed with
resistor R1, the base bias voltage is high enough to turn the transistor TR1 “ON” and thus
activate the relay which in turn is used to control some external circuitry. As the light level falls
back to darkness again the resistance of the LDR increases causing the base voltage of the
transistor to decrease, turning the transistor and relay “OFF” at a fixed light level determined
again by the potential divider network.
By replacing the fixed resistor R1 with a potentiometer VR1, the point at which the relay turns
“ON” or “OFF” can be pre-set to a particular light level. This type of simple circuit shown above
has a fairly low sensitivity and its switching point may not be consistent due to variations in
either temperature or the supply voltage. A more sensitive precision light activated circuit can be
easily made by incorporating the LDR into a “Wheatstone Bridge” arrangement and replacing
the transistor with an Operational Amplifier as shown.

Light Level Sensing Circuit

In this basic dark sensing circuit, the light dependent resistor LDR1 and the
potentiometer VR1 form one adjustable arm of a simple resistance bridge network, also known
commonly as a Wheatstone bridge, while the two fixed resistors R1 and R2 form the other arm.
Both sides of the bridge form potential divider networks across the supply voltage whose
outputs V1 and V2 are connected to the non-inverting and inverting voltage inputs respectively
of the operational amplifier.
The operational amplifier is configured as a Differential Amplifier also known as a voltage
comparator with feedback whose output voltage condition is determined by the difference
between the two input signals or voltages, V1 and V2. The resistor combination R1 and R2 form
a fixed voltage reference at input V2, set by the ratio of the two resistors. The LDR –
VR1 combination provides a variable voltage input V1 proportional to the light level being
detected by the photoresistor.
As with the previous circuit the output from the operational amplifier is used to control a relay,
which is protected by a free wheel diode, D1. When the light level sensed by the LDR and its
output voltage falls below the reference voltage set at V2 the output from the op-amp changes
state activating the relay and switching the connected load.
Likewise as the light level increases the output will switch back turning “OFF” the relay. The
hysteresis of the two switching points is set by the feedback resistor Rf can be chosen to give any
suitable voltage gain of the amplifier.
The operation of this type of light sensor circuit can also be reversed to switch the relay “ON”
when the light level exceeds the reference voltage level and vice versa by reversing the positions
of the light sensor LDR and the potentiometer VR1. The potentiometer can be used to “pre-set”
the switching point of the differential amplifier to any particular light level making it ideal as a
simple light sensor project circuit.

Photojunction Devices
Photojunction Devices are basically PN-Junction light sensors or detectors made from silicon
semiconductor PN-junctions which are sensitive to light and which can detect both visible light
and infra-red light levels. Photo-junction devices are specifically made for sensing light and this
class of photoelectric light sensors include the Photodiode and the Phototransistor.

The Photodiode.

Photo-diode

The construction of the Photodiode light sensor is similar to that of a conventional PN-junction
diode except that the diodes outer casing is either transparent or has a clear lens to focus the light
onto the PN junction for increased sensitivity. The junction will respond to light particularly
longer wavelengths such as red and infra-red rather than visible light.
This characteristic can be a problem for diodes with transparent or glass bead bodies such as the
1N4148 signal diode. LED’s can also be used as photodiodes as they can both emit and detect
light from their junction. All PN-junctions are light sensitive and can be used in a photo-
conductive unbiased voltage mode with the PN-junction of the photodiode always “Reverse
Biased” so that only the diodes leakage or dark current can flow.
The current-voltage characteristic (I/V Curves) of a photodiode with no light on its junction
(dark mode) is very similar to a normal signal or rectifying diode. When the photodiode is
forward biased, there is an exponential increase in the current, the same as for a normal diode.
When a reverse bias is applied, a small reverse saturation current appears which causes an
increase of the depletion region, which is the sensitive part of the junction. Photodiodes can also
be connected in a current mode using a fixed bias voltage across the junction. The current mode
is very linear over a wide range.
Photo-diode Construction and Characteristics

When used as a light sensor, a photodiodes dark current (0 lux) is about 10uA for geranium and
1uA for silicon type diodes. When light falls upon the junction more hole/electron pairs are
formed and the leakage current increases. This leakage current increases as the illumination of
the junction increases.
Thus, the photodiodes current is directly proportional to light intensity falling onto the PN-
junction. One main advantage of photodiodes when used as light sensors is their fast response to
changes in the light levels, but one disadvantage of this type of photodevice is the relatively
small current flow even when fully lit.
The following circuit shows a photo-current-to-voltage converter circuit using an operational
amplifier as the amplifying device. The output voltage (Vout) is given as Vout = Ip × Rf and
which is proportional to the light intensity characteristics of the photodiode.
This type of circuit also utilizes the characteristics of an operational amplifier with two input
terminals at about zero voltage to operate the photodiode without bias. This zero-bias op-amp
configuration gives a high impedance loading to the photodiode resulting in less influence by
dark current and a wider linear range of the photocurrent relative to the radiant light intensity.
Capacitor Cf is used to prevent oscillation or gain peaking and to set the output bandwidth
(1/2πRC).

Photo-diode Amplifier Circuit

Photodiodes are very versatile light sensors that can turn its current flow both “ON” and “OFF”
in nanoseconds and are commonly used in cameras, light meters, CD and DVD-ROM drives, TV
remote controls, scanners, fax machines and copiers etc, and when integrated into operational
amplifier circuits as infrared spectrum detectors for fibre optic communications, burglar alarm
motion detection circuits and numerous imaging, laser scanning and positioning systems etc.

The Phototransistor

Photo-transistor

An alternative photo-junction device to the photodiode is the Phototransistor which is basically

a photodiode with amplification. The Phototransistor light sensor has its collector-base PN-
junction reverse biased exposing it to the radiant light source.
Phototransistors operate the same as the photodiode except that they can provide current gain and
are much more sensitive than the photodiode with currents are 50 to 100 times greater than that
of the standard photodiode and any normal transistor can be easily converted into a
phototransistor light sensor by connecting a photodiode between the collector and base.
Phototransistors consist mainly of a bipolar NPN Transistor with its large base region
electrically unconnected, although some phototransistors allow a base connection to control the
sensitivity, and which uses photons of light to generate a base current which in turn causes a
collector to emitter current to flow. Most phototransistors are NPN types whose outer casing is
either transparent or has a clear lens to focus the light onto the base junction for increased
sensitivity.

Photo-transistor Construction and Characteristics

In the NPN transistor the collector is biased positively with respect to the emitter so that the
base/collector junction is reverse biased. therefore, with no light on the junction normal leakage
or dark current flows which is very small. When light falls on the base more electron/hole pairs
are formed in this region and the current produced by this action is amplified by the transistor.
Usually the sensitivity of a phototransistor is a function of the DC current gain of the transistor.
Therefore, the overall sensitivity is a function of collector current and can be controlled by
connecting a resistance between the base and the emitter but for very high sensitivity optocoupler
type applications, Darlington phototransistors are generally used.

Photo-darlington

Photodarlington transistors use a second bipolar NPN transistor to provide additional

amplification or when higher sensitivity of a photodetector is required due to low light levels or
selective sensitivity, but its response is slower than that of an ordinary NPN phototransistor.
Photo darlington devices consist of a normal phototransistor whose emitter output is coupled to
the base of a larger bipolar NPN transistor. Because a darlington transistor configuration gives a
current gain equal to a product of the current gains of two individual transistors, a
photodarlington device produces a very sensitive detector.
Typical applications of Phototransistors light sensors are in opto-isolators, slotted opto
switches, light beam sensors, fibre optics and TV type remote controls, etc. Infrared filters are
sometimes required when detecting visible light.
Another type of photojunction semiconductor light sensor worth a mention is the Photo-
thyristor. This is a light activated thyristor or Silicon Controlled Rectifier, SCR that can be
used as a light activated switch in AC applications. However their sensitivity is usually very low
compared to equivalent photodiodes or phototransistors.
To help increase their sensitivity to light, photo-thyristors are made thinner around the gate
junction. The downside to this process is that it limits the amount of anode current that they can
switch. Then for higher current AC applications they are used as pilot devices in opto-couplers to
switch larger more conventional thyristors.

Photovoltaic Cells.
The most common type of photovoltaic light sensor is the Solar Cell. Solar cells convert light
energy directly into DC electrical energy in the form of a voltage or current to a power a resistive
load such as a light, battery or motor. Then photovoltaic cells are similar in many ways to a
battery because they supply DC power.
However, unlike the other photo devices we have looked at above which use light intensity even
from a torch to operate, photovoltaic solar cells work best using the suns radiant energy.
Solar cells are used in many different types of applications to offer an alternative power source
from conventional batteries, such as in calculators, satellites and now in homes offering a form
of renewable power.

Photovoltaic Cell

Photovoltaic cells are made from single crystal silicon PN junctions, the same as photodiodes
with a very large light sensitive region but are used without the reverse bias. They have the same
characteristics as a very large photodiode when in the dark.
When illuminated the light energy causes electrons to flow through the PN junction and an
individual solar cell can generate an open circuit voltage of about 0.58v (580mV). Solar cells
have a “Positive” and a “Negative” side just like a battery.
Individual solar cells can be connected together in series to form solar panels which increases the
output voltage or connected together in parallel to increase the available current. Commercially
available solar panels are rated in Watts, which is the product of the output voltage and current
(Volts times Amps) when fully lit.
Characteristics of a typical Photovoltaic Solar Cell.

The amount of available current from a solar cell depends upon the light intensity, the size of the
cell and its efficiency which is generally very low at around 15 to 20%. To increase the overall
efficiency of the cell commercially available solar cells use polycrystalline silicon or amorphous
silicon, which have no crystalline structure, and can generate currents of between 20 to 40mA
per cm2.
Other materials used in the construction of photovoltaic cells include Gallium Arsenide, Copper
Indium Diselenide and Cadmium Telluride. These different materials each have a different
spectrum band response, and so can be “tuned” to produce an output voltage at different
wavelengths of light.
In this tutorial about Light Sensors, we have looked at several examples of devices that are
classed as Light Sensors. This includes those with and those without PN-junctions that can be
used to measure the intensity of light.
In the next tutorial we will look at output devices called Actuators. Actuators convert an
electrical signal into a corresponding physical quantity such as movement, force, or sound. One
such commonly used output device is the Electromagnetic Relay.
Home / Input/Output Devices / Electrical Relay

Electrical Relay
Thus far we have seen a selection of Input devices that can be used to detect or “sense” a variety
of physical variables and signals and are therefore called Sensors.
But there are also a variety of electrical and electronic devices which are classed
as Output devices used to control or operate some external physical process. These output
devices are commonly called Actuators.
Actuators convert an electrical signal into a corresponding physical quantity such as movement,
force, sound etc. An actuator is also classed as a transducer because it changes one type of
physical quantity into another and is usually activated or operated by a low voltage command
signal. Actuators can be classed as either binary or continuous devices based upon the number of
stable states their output has.
Related Products: Electromechanical Switches | Switch DIP | Switch Push Button
For example, a relay is a binary actuator as it has two stable states, either energised and latched
or de-energised and unlatched, while a motor is a continuous actuator because it can rotate
through a full 360o motion. The most common types of actuators or output devices are Electrical
Relays, Lights, Motors and Loudspeakers.
We saw previously that solenoids can be used to electrically open latches, doors, open or close
valves, and in a variety of robotic and mechatronic applications, etc. However, if the solenoid
plunger is used to operate one or more sets of electrical contacts, we have a device called
a relay that is so useful it can be used in an infinite number of different ways and in this tutorial
we will look at Electrical Relays.
Electrical Relays can also be divided into mechanical action relays called “Electromechanical
Relays” and those which use semiconductor transistors, thyristors, triacs, etc, as their switching
device called “Solid State Relays” or SSR’s.

The Electromechanical Relay

The term Relay generally refers to a device that provides an electrical connection between two
or more points in response to the application of a control signal. The most common and widely
used type of electrical relay is the electromechanical relay or EMR.
 An Electrical Relay
The most fundamental control of any equipment is the ability to turn it “ON” and “OFF”. The
easiest way to do this is using switches to interrupt the electrical supply. Although switches can
be used to control something, they have their disadvantages. The biggest one is that they have to
be manually (physically) turned “ON” or “OFF”. Also, they are relatively large, slow and only
switch small electrical currents.
Related Products: Switch Rocker | Switch Slide | Switch Thumb-Pushwheel
Electrical Relays however, are basically electrically operated switches that come in many
shapes, sizes and power ratings suitable for all types of applications. Relays can also have single
or multiple contacts within a single package with the larger power relays used for mains voltage
or high current switching applications being called “Contactors”.
In this tutorial about electrical relays we are just concerned with the fundamental operating
principles of “light duty” electromechanical relays we can use in motor control or robotic
circuits. Such relays are used in general electrical and electronic control or switching circuits
either mounted directly onto PCB boards or connected free standing and in which the load
currents are normally fractions of an ampere up to 20+ amperes. The relay circuit are common in
Electronics applications.
As their name implies, electromechanical relays are electro-magnetic devices that convert a
magnetic flux generated by the application of a low voltage electrical control signal either AC or
DC across the relay terminals, into a pulling mechanical force which operates the electrical
contacts within the relay. The most common form of electromechanical relay consist of an
energizing coil called the “primary circuit” wound around a permeable iron core.
This iron core has both a fixed portion called the yoke, and a moveable spring loaded part called
the armature, that completes the magnetic field circuit by closing the air gap between the fixed
electrical coil and the moveable armature. The armature is hinged or pivoted allowing it to freely
move within the generated magnetic field closing the electrical contacts that are attached to it.
Connected between the yoke and armature is normally a spring (or springs) for the return stroke
to “reset” the contacts back to their initial rest position when the relay coil is in the “de-
energized” condition, i.e. turned “OFF”.

Electromechanical Relay Construction

In our simple relay above, we have two sets of electrically conductive contacts. Relays may be
“Normally Open”, or “Normally Closed”. One pair of contacts are classed as Normally Open,
(NO) or make contacts and another set which are classed as Normally Closed, (NC) or break
contacts. In the normally open position, the contacts are closed only when the field current is
“ON” and the switch contacts are pulled towards the inductive coil.
In the normally closed position, the contacts are permanently closed when the field current is
“OFF” as the switch contacts return to their normal position. These terms Normally Open,
Normally Closed or Make and Break Contacts refer to the state of the electrical contacts when
the relay coil is “de-energized”, i.e, no supply voltage connected to the relay coil. Contact
elements may be of single or double make or break designs. An example of this arrangement is
given below.

The relays contacts are electrically conductive pieces of metal which touch together completing a
circuit and allow the circuit current to flow, just like a switch. When the contacts are open the
resistance between the contacts is very high in the Mega-Ohms, producing an open circuit
condition and no circuit current flows.
When the contacts are closed the contact resistance should be zero, a short circuit, but this is not
always the case. All relay contacts have a certain amount of “contact resistance” when they are
closed and this is called the “On-Resistance”, similar to FET’s.
With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ω’s
because the tips are new and clean, but over time the tip resistance will increase.
For example. If the contacts are passing a load current of say 10A, then the voltage drop across
the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts
then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they
are not properly protected from high inductive or capacitive loads, they will start to show signs
of arcing damage as the circuit current still wants to flow as the contacts begin to open when the
relay coil is de-energized.
This arcing or sparking across the contacts will cause the contact resistance of the tips to increase
further as the contact tips become damaged. If allowed to continue the contact tips may become
so burnt and damaged to the point were they are physically closed but do not pass any or very
little current.
If this arcing damage becomes to severe the contacts will eventually “weld” together producing a
short circuit condition and possible damage to the circuit they are controlling. If now the contact
resistance has increased due to arcing to say 1Ω’s the volt drop across the contacts for the same
load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be
unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty
relay will have to be replaced.
To reduce the effects of contact arcing and high “On-resistances”, modern contact tips are made
of, or coated with, a variety of silver based alloys to extend their life span as given in the
following table.

Electrical Relay Contact Tip Materials

 Ag (fine silver)
o 1. Electrical and thermal conductivity are the highest of all the metals.
o 2. Exhibits low contact resistance, is inexpensive and widely used.
o 3. Contacts tarnish easily through sulphurisation influence.
 AgCu (silver copper)
o 1. Known as “Hard silver” contacts and have better wear resistance and less tendency to
arc and weld, but slightly higher contact resistance.
 AgCdO (silver cadmium oxide)
o 1. Very little tendency to arc and weld, good wear resistance and arc extinguishing
properties.
 AgW (silver tungsten)
o 1. Hardness and melting point are high, arc resistance is excellent.
o 2. Not a precious metal.
 o 3. High contact pressure is required to reduce resistance.
o 4. Contact resistance is relatively high, and resistance to corrosion is poor.
 AgNi (silver nickel)
o 1. Equals the electrical conductivity of silver, excellent arc resistance.
 AgPd (silver palladium)
o 1. Low contact wear, greater hardness.
o 2. Expensive.
 Platinum, Gold and Silver Alloys
o 1. Excellent corrosion resistance, used mainly for low-current circuits.
Relay manufacturers data sheets give maximum contact ratings for resistive DC loads only and
this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order
to achieve long life and high reliability when switching alternating currents with inductive or
capacitive loads some form of arc suppression or filtering is required across the relay contacts.
Extending the life of relay tips by reducing the amount of arcing generated as they open is
achieved by connecting a Resistor-Capacitor network called an RC Snubber
Network electrically in parallel with an electrical relay contact tips. The voltage peak, which
occurs at the instant the contacts open, will be safely short circuited by the RC network, thus
suppressing any arc generated at the contact tips. For example.

Electrical Relay Snubber Circuit

Electrical Relay Contact Types.

As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used
to describe how the relays contacts are connected, relay contact arrangements can also be classed
by their actions. Electrical relays can be made up of one or more individual switch contacts with
each “contact” being referred to as a “pole”. Each one of these contacts or poles can be
connected or “thrown” together by energizing the relays coil and this gives rise to the description
of the contact types as being:
 SPST – Single Pole Single Throw
 SPDT – Single Pole Double Throw
 DPST – Double Pole Single Throw
 DPDT – Double Pole Double Throw
with the action of the contacts being described as “Make” (M) or “Break” (B). Then a simple
relay with one set of contacts as shown above can have a contact description of:
“Single Pole Double Throw – (Break before Make)”, or SPDT – (B-M)
Examples of just some of the more common diagrams used for electrical relay contact types to
identify relays in circuit or schematic diagrams is given below but there are many more possible
configurations.

Electrical Relay Contact Configurations

 Where:
 C is the Common terminal
 NO is the Normally Open contact
 NC is the Normally Closed contact
Electromechanical relays are also denoted by the combinations of their contacts or switching
elements and the number of contacts combined within a single relay. For example, a contact
which is normally open in the de-energised position of the relay is called a “Form A contact” or
make contact. Whereas a contact which is normally closed in the de-energised position of the
relay is called a “Form B contact” or break contact.
When both a make and a break set of contact elements are present at the same time so that the
two contacts are electrically connected to produce a common point (identified by three
connections), the set of contacts are referred to as “Form C contacts” or change-over contacts. If
no electrical connection exists between the make and break contacts it is referred to as a double
change-over contact.
One final point to remember about using electrical relays. It is not advisable at all to connect
relay contacts in parallel to handle higher load currents. For example, never attempt to supply a
10A load with two relay contacts in parallel that have 5A contact ratings each, as the
mechanically operated relay contacts never close or open at exactly the same instant of time. The
result is that one of the contacts will always be overloaded even for a brief instant resulting in
premature failure of the relay over time.
Also, while electrical relays can be used to allow low power electronic or computer type circuits
to switch relatively high currents or voltages both “ON” or “OFF”. Never mix different load
voltages through adjacent contacts within the same relay such as for example, high voltage AC
(240v) and low voltage DC (12v), always use separate relays for safety.
One of the more important parts of any electrical relay is its coil. This converts electrical current
into an electromagnetic flux which is used to mechanically operate the relays contacts. The main
problem with relay coils is that they are “highly inductive loads” as they are made from coils of
wire. Any coil of wire has an impedance value made up of resistance ( R ) and inductance ( L ) in
series (LR Series Circuit).
As the current flows through the coil a self induced magnetic field is generated around it. When
the current in the coil is turned “OFF”, a large back emf (electromotive force) voltage is
produced as the magnetic flux collapses within the coil (transformer theory). This induced
reverse voltage value may be very high in comparison to the switching voltage, and may damage
any semiconductor device such as a transistor, FET or micro-controller used to operate the relay
coil.

One way of preventing damage to the transistor or any switching semiconductor device, is to
connect a reverse biased diode across the relay coil.
When the current flowing through the coil is switched “OFF”, an induced back emf is generated
as the magnetic flux collapses in the coil.
This reverse voltage forward biases the diode which conducts and dissipates the stored energy
preventing any damage to the semiconductor transistor.
When used in this type of application the diode is generally known as a Flywheel Diode, Free-
wheeling Diode and even Fly-back Diode, but they all mean the same thing. Other types of
inductive loads which require a flywheel diode for protection are solenoids, motors and inductive
coils.
As well as using flywheel Diodes for protection of semiconductor components, other devices
used for protection include RC Snubber Networks, Metal Oxide
Varistors or MOV and Zener Diodes.

The Solid State Relay.

While the electromechanical relay (EMR) are inexpensive, easy to use and allow the switching
of a load circuit controlled by a low power, electrically isolated input signal, one of the main
disadvantages of an electromechanical relay is that it is a “mechanical device”, that is it has
moving parts so their switching speed (response time) due to physically movement of the metal
contacts using a magnetic field is slow.
Over a period of time these moving parts will wear out and fail, or that the contact resistance
through the constant arcing and erosion may make the relay unusable and shortens its life. Also,
they are electrically noisy with the contacts suffering from contact bounce which may affect any
electronic circuits to which they are connected.
To overcome these disadvantages of the electrical relay, another type of relay called a Solid
State Relay or (SSR) for short was developed which is a solid state contactless, pure electronic
relay.
The solid state relay being a purely electronic device has no moving parts within its design as the
mechanical contacts have been replaced by power transistors, thyristors or triac’s. The electrical
separation between the input control signal and the output load voltage is accomplished with the
aid of an opto-coupler type Light Sensor.
The Solid State Relay provides a high degree of reliability, long life and reduced
electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a
much faster almost instant response time, as compared to the conventional electromechanical
relay.
Also the input control power requirements of the solid state relay are generally low enough to
make them compatible with most IC logic families without the need for additional buffers,
drivers or amplifiers. However, being a semiconductor device they must be mounted onto
suitable heatsinks to prevent the output switching semiconductor device from over heating.

Solid State Relay

The AC type Solid State Relay turns “ON” at the zero crossing point of the AC sinusoidal
waveform, prevents high inrush currents when switching inductive or capacitive loads while the
inherent turn “OFF” feature of Thyristors and Triacs provides an improvement over the arcing
contacts of the electromechanical relays.
Like the electromechanical relays, a Resistor-Capacitor (RC) snubber network is generally
required across the output terminals of the SSR to protect the semiconductor output switching
device from noise and voltage transient spikes when used to switch highly inductive or
capacitive loads. In most modern SSR’s this RC snubber network is built as standard into the
relay itself reducing the need for additional external components.
Non-zero crossing detection switching (instant “ON”) type SSR’s are also available for phase
controlled applications such as the dimming or fading of lights at concerts, shows, disco lighting
etc, or for motor speed control type applications.
As the output switching device of a solid state relay is a semiconductor device (Transistor for DC
switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop
across the output terminals of an SSR when “ON” is much higher than that of the
electromechanical relay, typically 1.5 – 2.0 volts. If switching large currents for long periods of
time an additional heat sink will be required.

Input/Output Interface Modules.

Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed
specifically to interface computers, micro-controller or PIC’s to “real world” loads and switches.
There are four basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS
logic level output, and TTL or CMOS logic input to an AC or DC Output voltage with each
module containing all the necessary circuitry to provide a complete interface and isolation within
one small device. They are available as individual solid state modules or integrated into 4, 8 or
16 channel devices.

Modular Input/Output Interface System.

The main disadvantages of solid state relays (SSR’s) compared to that of an equivalent wattage
electromechanical relay is their higher costs, the fact that only single pole single throw (SPST)
types are available, “OFF”-state leakage currents flow through the switching device, and a high
“ON”-state voltage drop and power dissipation resulting in additional heat sinking requirements.
Also they can not switch very small load currents or high frequency signals such as audio or
video signals although special Solid State Switches are available for this type of application.
In this tutorial about Electrical Relays, we have looked at both the electromechanical relay and
the solid state relay which can be used as an output device (actuator) to control a physical
process. In the next tutorial we will continue our look at output devices called Actuators and
especially one that converts a small electrical signal into a corresponding physical movement
using electromagnetism. The output device is called a Solenoid.
Home / Input/Output Devices / Linear Solenoid Actuator

Linear Solenoid Actuator

Another type of electromagnetic actuator that converts an electrical signal into a magnetic field
producing a linear motion is called the Linear Solenoid.
The linear solenoid works on the same basic principal as the electromechanical relay seen in the
previous tutorial and just like relays, they can also be switched and controlled using transistors or
MOSFET’s. A “Linear Solenoid” is an electromagnetic device that converts electrical energy
into a mechanical pushing or pulling force or motion.

Linear Solenoid
Linear solenoid’s basically consist of an electrical coil wound around a cylindrical tube with a
ferro-magnetic actuator or “plunger” that is free to move or slide “IN” and “OUT” of the coils
body. Solenoids can be used to electrically open doors and latches, open or close valves, move
and operate robotic limbs and mechanisms, and even actuate electrical switches just by
energising its coil.
Solenoids are available in a variety of formats with the more common types being the linear
solenoid also known as the linear electromechanical actuator, (LEMA) and the rotary solenoid.
Both types of solenoid, linear and rotational are available as either a holding (continuously
energised) or as a latching type (ON-OFF pulse) with the latching types being used in either
energised or power-off applications. Linear solenoids can also be designed for proportional
motion control were the plunger position is proportional to the power input.
When electrical current flows through a conductor it generates a magnetic field, and the direction
of this magnetic field with regards to its North and South Poles is determined by the direction of
the current flow within the wire. This coil of wire becomes an “Electromagnet” with its own
north and south poles exactly the same as that for a permanent type magnet.
The strength of this magnetic field can be increased or decreased by either controlling the
amount of current flowing through the coil or by changing the number of turns or loops that the
coil has. An example of an “Electromagnet” is given below.

Magnetic Field produced by a Coil

When an electrical current is passed through the coils windings, it behaves like an electromagnet
and the plunger, which is located inside the coil, is attracted towards the centre of the coil by the
magnetic flux setup within the coils body, which in turn compresses a small spring attached to
one end of the plunger. The force and speed of the plungers movement is determined by the
strength of the magnetic flux generated within the coil.
When the supply current is turned “OFF” (de-energised) the electromagnetic field generated
previously by the coil collapses and the energy stored in the compressed spring forces the
plunger back out to its original rest position. This back and forth movement of the plunger is
known as the solenoids “Stroke”, in other words the maximum distance the plunger can travel in
either an “IN” or an “OUT” direction, for example, 0 – 30mm.

Linear Solenoid Construction

This type of solenoid is generally called a Linear Solenoid due to the linear directional
movement and action of the plunger. Linear solenoids are available in two basic configurations
called a “Pull-type” as it pulls the connected load towards itself when energised, and the “Push-
type” that act in the opposite direction pushing it away from itself when energised. Both push
and pull types are generally constructed the same with the difference being in the location of the
return spring and design of the plunger.

Pull-type Linear Solenoid Construction

Linear solenoids are useful in many applications that require an open or closed (in or out) type
motion such as electronically activated door locks, pneumatic or hydraulic control valves,
robotics, automotive engine management, irrigation valves to water the garden and even the
“Ding-Dong” door bell has one. They are available as open frame, closed frame or sealed tubular
types.

Rotary Solenoids
Most electromagnetic solenoids are linear devices producing a linear back and forth force or
motion. However, rotational solenoids are also available which produce an angular or rotary
motion from a neutral position in either clockwise, anti-clockwise or in both directions (bi-
directional).

Rotary Solenoid
Rotary solenoids can be used to replace small DC motors or stepper motors were the angular
movement is very small with the angle of rotation being the angle moved from the start to the
end position.
Commonly available rotary solenoids have movements of 25, 35, 45, 60 and 90o as well as
multiple movements to and from a certain angle such as a 2-position self restoring or return to
zero rotation, for example 0-to-90-to-0o, 3-position self restoring, for example 0oto +45o or 0o to -
45o as well as 2-position latching.
Rotary solenoids produce a rotational movement when either energised, de-energised, or a
change in the polarity of an electromagnetic field alters the position of a permanent magnet rotor.
Their construction consists of an electrical coil wound around a steel frame with a magnetic disk
connected to an output shaft positioned above the coil.
When the coil is energised the electromagnetic field generates multiple north and south poles
which repel the adjacent permanent magnetic poles of the disk causing it to rotate at an angle
determined by the mechanical construction of the rotary solenoid.
Rotary solenoids are used in vending or gaming machines, valve control, camera shutter with
special high speed, low power or variable positioning solenoids with high force or torque are
available such as those used in dot matrix printers, typewriters, automatic machines or
automotive applications etc.

Solenoid Switching
Generally solenoids either linear or rotary operate with the application of a DC voltage, but they
can also be used with AC sinusoidal voltages by using full wave bridge rectifiers to rectify the
supply which then can be used to switch the DC solenoid. Small DC type solenoids can be easily
controlled using Transistor or MOSFET switches and are ideal for use in robotic applications.
However, as we saw previously with electromechanical relays, linear solenoids are “inductive”
devices so some form of electrical protection is required across the solenoid coil to prevent high
back emf voltages from damaging the semiconductor switching device. In this case the standard
“Flywheel Diode” is used, but you could equally use a zener diode or small value varistor.

Switching Solenoids using a Transistor

Reducing Energy Consumption

One of the main disadvantages of solenoids and especially the linear solenoid is that they are
“inductive devices”. This means that their solenoid coil converts some of the electrical energy
used to operate them into “HEAT”.
In other words when connected for long periods of time to an electrical supply they get hot!, and
the longer the time that the power is applied to a solenoid coil, the hotter the coil will become.
Also as the coil heats up, its electrical resistance also changes allowing more current to flow
increasing its temperature.
With a continuous voltage input applied to the coil, the solenoids coil does not have the
opportunity to cool down because the input power is always on. In order to reduce this self
generated heating effect it is necessary to reduce either the amount of time the coil is energised
or reduce the amount of current flowing through it.
One method of consuming less current is to apply a suitable high enough voltage to the solenoid
coil so as to provide the necessary electromagnetic field to operate and seat the plunger but then
once activated to reduce the coils supply voltage to a level sufficient to maintain the plunger in
its seated or latched position. One way of achieving this is to connect a suitable “holding”
resistor in series with the solenoids coil, for example:

Reducing Solenoid Energy Consumption

Here, the switch contacts are closed shorting out the resistance and passing the full supply
current directly to the solenoid coils windings. Once energised the contacts which can be
mechanically connected to the solenoids plunger action open connecting the holding
resistor, RH in series with the solenoids coil. This effectively connects the resistor in series with
the coil.
By using this method, the solenoid can be connected to its voltage supply indefinitely
(continuous duty cycle) as the power consumed by the coil and the heat generated is greatly
reduced, which can be up to 85 to 90% using a suitable power resistor. However, the power
consumed by the resistor will also generate a certain amount of heat, I2R (Ohm’s Law) and this
also needs to be taken into account.

Solenoid Duty Cycle

Another more practical way of reducing the heat generated by the solenoids coil is to use an
“intermittent duty cycle”. An intermittent duty cycle means that the coil is repeatedly switched
“ON” and “OFF” at a suitable frequency so as to activate the plunger mechanism but not allow it
to de-energise during the OFF period of the waveform. Intermittent duty cycle switching is a
very effective way to reduce the total power consumed by the coil.
The Duty Cycle (%ED) of a solenoid is the portion of the “ON” time that a solenoid is energised
and is the ratio of the “ON” time to the total “ON” and “OFF” time for one complete cycle of
operation. In other words, the cycle time equals the switched-ON time plus the switched-OFF
time. Duty cycle is expressed as a percentage, for example:

Then if a solenoid is switched “ON” or energised for 30 seconds and then switched “OFF” for 90
seconds before being re-energised again, one complete cycle, the total “ON/OFF” cycle time
would be 120 seconds, (30+90) so the solenoids duty cycle would be calculated as 30/120 secs or
25%. This means that you can determine the solenoids maximum switch-ON time if you know
the values of duty cycle and switch-OFF time.
For example, the switch-OFF time equals 15 secs, duty cycle equals 40%, therefore switch-ON
time equals 10 secs. A solenoid with a rated Duty Cycle of 100% means that it has a continuous
voltage rating and can therefore be left “ON” or continuously energised without overheating or
damage.
In this tutorial about solenoids, we have looked at both the Linear Solenoid and the Rotary
Solenoid as an electromechanical actuator that can be used as an output device to control a
physical process. In the next tutorial we will continue our look at output devices
called Actuators, and one that converts a electrical signal into a corresponding rotational
movement again using electromagnetism. The type of output device we will look at in the next
tutorial is the DC Motor.
http://www.electronics-tutorials.ws/ for more details