Experiment #9

Photo-coupler and Touch Alarm Switch

An Opto-coupler (photo-coupler), is an electronic components that interconnects two separate electrical
circuits by means of a light sensitivity optical interface.
Phototransistor Opto-coupler:

Assume a photo-transistor device as shown. Current from the source signal passes through the
input LED which emits an infra-red light whose intensity is proportional to the electrical signal

This emitted light falls upon the base of the photo-transistor, causing it to switch ON and conduct
in a similar way to a normal bipolar transistor.

The base connection of the photo-transistor can be left open (unconnected) for maximum
sensitIvity to the LEDs Infra-red light energy or connected to ground via suitable external high
value resistor to control the switching sensitivity making it more stable and resistant to false
triggering by external electrical noise or voltage transients.

When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off
causing the photo-transistor to cease conducting. The photo-transistor can be used to switch current
in the output circuit. The spectral response of the LED and the photosensitive device are closely
matched being separated by a transparent medium such a glass, plastic or air. Since there is no
direct electrical connection between the input and output of an optocoupler, electrical isolation up
to 10KV is achieved.

Optocouplers are available in four general types, each one having an infrared LED source but
with different photo-sensitive devices. The four optocouplers are called the: Photo-transistor
Photo-darlington, Photo-SCR and Photo-trine as shown below.
Optocoupler Applications:
Optocouplers and opto-isolators can be used on their own, or to switch a range of other larger
electronic devices such as transistors and TRIAC’s providing the required electrical isolation between a
lower voltage control signal, for example one from an Arduino or microcontroller, and a much higher
voltage or mains current output signal.

Common applications for opto-couplers include microprocessor input output switching. DC and AC
power control. PC communications, signal isolation and power supply regulation which suffer from
current ground loops, etc. The electrical signal being transmitted can be either analogue (linear) or
digital (pulses).

In this application, the optocoupler is used to detect the operation of the switch or another type of
digital input signal. This is useful if the switch or signal being detected is within an electrically noisy
environment. The output can be used to operate an external circuit, light or as an input to a PC or
microprocessor.

Touch Alarm switch

A touch switch is a type of switch that only has to be touched by an object to operate. It is used in many
lamps and wall switches that have a metal exterior as well as on public computer terminals, touch
screen includes an array of touch switches on a display. A touch switch is the simplest kind of tactile
sensor.

Touch the sensor of the alarm with your finger and it starts beeping, goes on for some time and then
stops. Touching it again, and it goes again! This little and flexible circuit consists of touch sensor and a
directly coupled transistor amplifier with a small buzzer as the output load.
Types of touch switches:
• Capacitance switch (Capacitive Touch)
A capacitance switch needs only one electrode to function. The electrode can be placed behind a non-
conductive panel such as wood, glass. or plastic. The switch works using body capacitance. A property of
the human body that gives it great electrical characteristics. The switch keeps charging and discharging
its metal exterior to detect changes in capacitance. When a person touches it, their body increases the
capacitance and triggers the switch.

Capacitance switches are available commercially as integrated circuits from a number of

manufacturers. These devices can also be used as a short-range proximity sensor.

• Resistance touch switch (Resistive Touch)

A resistance switch needs two electrodes to be physically in contact with something electrically
conductive (for example a finger) to operate. They work by lowering the resistance between two pieces
of metal. It is thus much simpler in construction compared to the capacitance switch. Placing one or two
fingers across the plates achieves a turn on or closed state. Removing the finger(s) from the metal pieces
turns the device off.

• Piezo touch switch

Piezo touch switches are based on mechanical bending of piezo ceramic, typically constructed directly
behind a surface. This solution enables touch interfaces with any kind of material. Another characteristic
of piezo is that it can function as
actuator as well. Current commercial solutions construct the piezo in such a way that touching it with
approximately 1.5 N is enough, even for stiff materials like stainless steel.

Piezo touch switches are available commercially.

EXPERIMENT

Photo-Coupler and Touch Alarm Circuits

OBJECTIVE
1. Understanding the characteristics of photocouplers.
2. Understanding the characteristics of FETs.
3. Performing the photocoupler control circuit
4. Performing the FET touch alarm circuit.

DISCUSSION
Photocoupler

Light emitting devices and light sensing devices have major applications in areas where electrical
isolation between the input signal and the output is important. Fig. 17-1 shows the appearance and
circuit symbol of a photo-coupler, optical isolator, or phototransistor coupled pair.

The advantages of photocouplers over relays and transformers are:

(1) Low Cost
(2) Small size and light weight
(3) High speed switching with bounceless
(4) No contact spikes
The operation of the photo-coupler can be considered as a communication system as shown in Fig. 17-2.
When an input signal is applied to the light emitting diode (LED) the light emitted is detected by the
phototransistor and converted back to an electrical signal.

The photocoupler is widely used as an interface between two different voltage levels. Fig.17-3 shows
the applications for the conversion between high voltage Indicator and low voltage signal. In each of
these two circuits, the electrical Isolation between high voltage signal and low voltage signal is excellent.

The resistor R the circuit of Fig 17-3(a) is used to limit the current flow in lamp. When the switch is
opened, the lamp extinguishes since no voltage applied. The resistance of photoconductor increases and
drives the transistor to conduct into saturation. Therefore, the output voltage is 0. When the switch is
closed, the lamp lights up. The resistance of photo conductor decreases and causes the transistor to cut
off. The output voltage equal to Vcc.

Due to the photocoupler is suited for AC or DC signals, it is also called the universal signal transformer.
The most popular type of photocouplers consisting of an LED and a phototransistor is shown in Fig. 17-
3(b). When the positive voltage is applied to LED, the light emitted is detected by the phototransistor
and converted back to an electrical signal.

The light emitting diode is p-n junction which when forward biased will emit light. The phototransistor
can operate in extremely high response. There are several inherent advantages of an LED-
phototransistor combination over conventional light sources and detectors.
The advantages of the circuit of Fig. 17-3(b) over the circuit of Fig. 17-3(a) are:

(1) Long life - The life of LED is longer than any types of lam bulb (10000-hour typical).
(2) High shock and vibration immunity - These features make LED-phototransistor combination to suit
for industrial control applications
(3) High speed-LED-phototransistor combination is suited in the application of high frequency switching

Figs. 17-4 shows the characteristics of a photocoupler.

The region to the left of the dashed curve in Fig. 17-7 is replotted to an expanded scale in Fig. 17-9 and
the curves are extended into the third quadrant. The resistance at the origin is the reciprocal of the
slope of the curves at the origin. The slope of the curve for the gate voltage equal to pinch off voltage (-
3V) is zero, and the corresponding value for off resistance Ron is infinite or an open circuit. The curve for
zero gate voltage yields a value for on resistance Ron of several hundred ohms. In this region, the JFET is
useful as a voltage-controlled variable-resistance ( VVR) for the applications of automatic gain control
(AGC) and switch circuits.

Testing FET with Ohmmeter

To identify the terminals of a JFET with an ohmmeter, the following steps useful:

1. Set range selector of ohmmeter at R x 1K range. Measure the junction resistance either G-to-D
or G-to-S to find the gate terminal. Assuming an N-channel JFET under testing, connect the black lead
(battery positive) of ohmmeter to the gate (G) and the red lead (battery negative) to either D or S, the
resistance indication should be low. If a P-channel JFET is tested, reverse the leads of ohmmeter.
2. If the range selector of ohmmeter is set at Rx1, some troubles may be encountered in the
measurement step 1 This is caused by the difference of p-n forward characteristics between JFET and
conventional transistor as shown in Fig. 17-10. The forward characteristic of conventional transistor or
diode is that the forward Voltage drop holds between 0.6V and 0.7V once the forward current flowing.
The p-n junction characteristic of JFET is like a diode series with a resistor. In other words, the junction
resistance of JFET is greater than that of a transistor. Therefore, a high resistance range of ohmmeter
should be used.

3. The resistance of drain-to-source should be several hundred ohms either forward or reverse.
Assume an N-channel JFET under testing. Set the range selector of ohmmeter to low resistance range
Connect the black lead of ohmmeter to the terminal D or S and the red lead to the other terminal. With
your finger, touch the black lead and terminal G simultaneously and record the resistance reading.
Reverse the leads and repeat the measurement. Comparing these two results, the measurement of low
reading is proper bias arrangement. That is, the terminal with the black lead is terminal D and the
terminal with the red lead is terminal S.

Description of Experiment Circuit

Fig. 17-11 shows the experiment circuit. It consists of photo-coupler control circuit and touch alarm
circuit. The operation of each circuit is described as follows.
1. Photo-coupler Control Circuit
Transistor Q1 and photo-coupler form the photo-coupler controller. Transistors Q3 and 04 and relay
form the control circuit. When DC 5V is applied to the base of Q1, the collector current of Q1 drives the
LED of the photo-coupler to light and the phototransistor conducts. The voltage across R3 drives both
Q3 and Q4 to conduct. Thus relay is energized and LED2 lights.

When the base of Q1 is connected to OV, Q1 off and phototransistor off result in Q3
off and Q4 off. Hence the relay is not energized and LED1 is on.

2. Touch Alarm Circuit

FET Q2 acts as a touch switch. Transistors Q3 and Q4 and buzzer for alarm circuit. Since the input
impedance of FET is extremely high, the sensitivity of the gate is very high. The maximum drain current
occurs at VGs=OV. When the TOUCH point is open, resistor R4 provides a bias for the FET. The drain
current lD flows through R5 resulting in a large voltage drop across R5. The result is the drain voltage at
low potential to force Q3 and Q4 off. Therefore, the buzzer is off. If a finger touches the TOUCH point,
the induced signal forces FET to off and hence the drain voltage rises to high voltage. This voltage drives
Q3 and 04 to conduct. Hence the buzzer sounds. When the finger is removed from TOUCH point, the
circuit recovers its initial state and buzzer returns off.

EQUIPMENT REQUIRED
1 — Power Supply Unit KL-51001
1 — Isolation Transformer KL-58002
1 — Module KL-53008
1 — Function generator (Optional)
1 — Multimeter

PROCEDURE

1. Connect 5V and 12V DC supplies from Power Supply Unit KL-51001 KL-58002 to Module
KL-53008.

2. Set SW to OFF position. Insert connect plugs in positions 1, 2, 5, and 7.

3. Which of LEDs is on? _______________________________________________________

is the relay energized? _____________________________________________________
does this circuit operate normally? ___________________________________________
4. Using the multi-meter, measure and record the voltages at Q1 collector, photo-coupler
E, Q3 collector, and Q4 collector.
VC1 = _____________________ VE1 = _______________________
VC3 = ____________________ VC4 =______________________

5. Set SQ to ON position. Is the relay activated? __________________________

Which of the LEDs is on? ___________________________________________
Does the photo-coupler circuit operate normally?
___________________________________________________________________

6. Repeat step 4.
VC1 = _________________________, VE1 = ________________________
VC3 = _________________________, VC4 = ________________________

7. If there is a function generator beside you, please do following additional experiment.

Set the output of function generator to 1Hz. 5VP-P in TTL level and connect DC 5V
terminal on Module KL-53008. Observe and record the operation of this circuit, states of
relay and LEDs.
________________________________________________________________________
_________

8. Remove all connect plugs from the Module. Insert connect plugs in positions 3, 4, and 6.
Does the buzzer sound?
________________________________________________________________________
_________
 9. Using the multimeter, measure and record the voltages at FET drain, Q3 collector, and
Q4 collector.
VD= _______________________________________________
VC3 = _______________________, VC4 = _______________________________

10. Touch the terminal "TOUCH" with your finger. Does the buzzer sound?
________________________________________________________________________
_________

Using the multimeter, measure and record the voltages at FET drain, Q3 collector, and
Q4 collector.
VD= _______________________________________________
VC3 = _______________________, VC4 = _______________________________

11. Remove your finger from the “TOUCH” terminal. Does the buzzer sound?
________________________________________________________________

Does the touch alarm circuit operate normally?

________________________________________________________________________
_________

CONCLUSION

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