DE2901940A1 - Photosensitive detector for paper document format - has light sensor and comparator with network applying variable reference voltage - Google Patents

Abstract

The paper document format detector has a light sensor (2) connected to a voltage source (1), whose internal resistance changes with the received light. A comparator (7) is in the form of a differential amplifier with an inverting (6) and non-inverting (8) inputs. A feed-back (11) leads to the non-inverting input (8). A connection is made between one of the inputs (6, 8) and the sensor (2). Resistors (12-14) and a first capacitor (15) between the comparator inputs and frame deliver a variable reference voltage, depending on the signal voltage, to one of the inputs.

Description

The present invention relates to a photosensitive

Detector according to the preamble of claim 1, wherein such Detectors for automatic format handling in printers, terminals of data processing systems and similar devices. It is known that automatic format handling to a large extent in terminals of data processing systems and in printers used to store checks, bills, passbooks, invoice forms, etc.

position automatically.

There are generally documents or printing documents with different Size, i.e. with a different format, as well as with different light transmittance and thickness used. Such documents are generally top-down introduced into a suitable carrier, which is provided with guide elements, until the lower edge of the documents is in contact with a stop and adjustment member. The device then automatically removes the sopping member and the document further introduced into the carrier by means of rollers or other drive devices, until the top of the document is in a predetermined position.

An effective and reliable device actually used Determining the position of a document includes a light source and a light source Photodetector in the movement path of the document during its positioning and subsequent handling. If the writing is not introduced into the establishment the light source illuminates the photodetector. In general, this Do not check the condition again if the document has been inserted into the carrier by hand is and is held by the stop and adjustment member. In this case it is located the document moves between the light source and the photodetector, and it becomes an electrical signal of a first type is supplied. This signal can be generated by the Data processing system used to make the subsequent automatic Activate positioning. When the introduction of the document has been carried out, this operation depending on the length of the document, illuminates the The light source again delivers the photodetector, thereby delivering another electrical signal which can be used by the data processing system, for example, to stop the movement of the document or up to a predetermined one Amount to continue. The automatic format handling device starts at this point a return of the document, generally from printing and reading operations, respectively is accompanied so that the document is again with its upper edge in the way of the light is brought between the light source and the photodetector device, whereby a new control signal is generated for the system.

The signals used in the system are therefore sensitive to changes in the Illumination of the photodetector in the form of light / dark transitions or vice versa traced back. The photodetector is usually connected to a threshold discrimination circuit connected, which makes it possible to determine whether the lighting level is higher or is lower than a predetermined level. Depending on the input signal the discrimination circuit provides an output signal corresponding to the Logic levels 1 or 0 can have two possible electrical values.

The amplitude of the signal of the output of the photodetector can be great Measure, on the other hand, through changes in the luminosity of the light source or the sensitivity of the photodetector or by the presence of opaque particles and Dust between the light source and the photodetector can be affected. Using a Discrimination circuit with a fixed threshold value can here for the case lead to a false display if the signal corresponds to a maximum lighting state is reduced to the threshold value for the reasons mentioned above. on the other hand it is not possible to set the threshold degrading without creating of a critical state with regard to the differentiation of the signals to risk the minimal illumination of the photodetector.

In IT-PS 900 533 there is a light-sensitive detector and an associated one Circuit shown and described, the threshold level being automatic is changed depending on the maximum lighting state. Such Device is partially suitable for use in connection with punch card readers, where the opacity of the punch card is ensured. The establishment however, it is not very suitable for the automatic handling of documents, though the documents have a different light transmittance It is therefore the object of the present invention, a light-sensitive detector of the opening to improve said type so that a correct threshold value depending on the transparency of the document mer is adjusted.

This problem is solved according to S Xer characterized in claim 1 Invention. Further advantageous refinements of the invention are set out in the subclaims removable.

The light-sensitive detector for documents according to the invention allows the threshold value to be changed automatically depending on the Properties of the documents handled.

Another advantage of the detector according to the invention is that that it is extremely sensitive to both transitions from a stronger lighting state to a weaker lighting state as well as for reversed transitions independently from the maximum and minimum lighting level, respectively. This is according to the invention achieved by using a comparison circuit with two inputs, one input being the input signal and the other input being a reference signal is fed. The reference signal is held at a level that is slightly different differs from the signal level.

Based on the figures of the accompanying drawing, an exemplary embodiment is provided of the invention described in more detail below. They show: Fig. 1 a light-sensitive Document detector with an associated differential threshold circuit according to the invention; FIG. 2 shows an electrically equivalent circuit for part of the device according to FIG Fig. 1 in a first operating state of this device; Fig. 3 an electrical equivalent circuit for part of the device according to FIG. 1 in a second Operating condition of this facility; 4 shows an electrically equivalent circuit for part of the device according to FIG. 1 in a third operating state of this Facility; Fig. 5.6 shows the level changes of the input signal over time, of the reference signal and the output signal of the device according to FIG. 1 for different Operating states.

Referring to Fig. 1, there is a photosensitive document detector and an associated differential threshold circuit is shown.

The photosensitive detector comprises a light source 1 and a Phototransistor 2, which are arranged opposite one another and one on both sides Are located level, which is shown by a dashed line 3. Along the Line 3 is a document 4 is moved, the movement being through a not shown automatic handling device takes place. If the document 4 is between the Light source and the photodetector is moved through, it becomes the photodetector emitted light beam interrupted. The collector of phototransistor 2 is over a resistor 5 with a suitable value of for example 10R to a suitable one Voltage source V of, for example, + 5V connected.

The emitter of the phototransistor 2 is connected to an inverting input 6 of a comparison circuit 7 connected, wherein this inverting In this case the input forms the signal input. The comparison circuit 7 can consist for example of the integrated module LM339 from the company NATIONAL is distributed.

It is not here on the constructional details of such Circuit received, which can be found in the manufacturer's catalog, but it should only be noted for the understanding of the invention that the comparison circuit 7 has two inputs 6 and 8 and one output 9.

If the voltage at the non-inverting input 8 is slightly higher than the voltage at the inverting input 6, the output 9 takes one High impedance state on, i.e. this output is internally isolated so that it has a Assumes voltage given by external circuit connections. if the voltage at the inverting input 6 is slightly greater than the voltage is at input 8, output 9 is grounded by comparison circuit 7. The two inputs 6 and 8 can be used as signal inputs and independently of each other Reference voltage input can be used. Both inputs have an infinitely high Input impedance.

The photosensitive detector device is passive by some Elements completed. A resistor 10 with a suitable value of, for example 1OkR connects the output 9 to a suitable voltage source V of for example + 5V. Another resistor 11 with a suitable value of, for example, 1252 connects output 9 to input 8, creating a positive feedback will. A third resistor 12 with a suitable value of, for example, 2ki connects the two entrances with each other. A fourth resistor 13 to which a variable resistor 14 is connected in series, connects the input 6 with Dimensions. Suitable values for resistors 13 and 14 are 1ki2 and a value between 0 and 5k; 2.

A capacitor 15 with a relatively large capacitance of, for example 10 FF is also connected between input 8 and ground, while a capacitor 16 with a limited value of e.g. 10 nF between input 6 and Ground is switched.

The operation of the circuit is very simple. In a first Operating state, which can be defined as the idle state, whereby the document is not located between the light source and the phototransistor, which is through the Phototransistor 5 the signal input 6 impressed voltage sufficiently high to the Output 9 of the comparison circuit 7 to ground. In this case the phototransistor works 5 as a conductive element with a low resistance of, for example, 2 k52 .

Fig. 2 shows the equivalent input circuit of the comparator 7 for this state, with the corresponding resistance values for the various elements are written. It is very easy to calculate that in this state a Voltage of approximately 2.93 to 3V is fed to input 6, while a voltage of approximately 2.51V is present at input 8. In this case the threshold is about 2.5V, and it should be noted that even in the case of fluctuations in the Supply voltage or the lighting state this voltage for a certain Duration is kept constant due to the effect of the connected to input 8 Capacitor 15.

If now due to a transfer of a document between Light source and phototransistor changed the lighting state of the phototransistor becomes, the internal resistance of the phototransistor 2 increases and it is immediate it can be seen that1 as soon as this slightly exceeds a value of 4 kS2, the voltage supplied to the input 6 drops below the threshold value and the comparator 7 toggles. The exit 9 of the comparator 7 is then isolated and pulled through the resistor 10 to a positive voltage level.

A positive voltage feedback occurs across resistor 11, whereby the capacitor 15 is charged further and a certain voltage between at entrances 6 and 8.

This voltage causes the comparator to enter a new stabilized one State. This state can be defined as an active state.

When the new stabilized state is reached, the electrical Behavior of the device in a simple manner on the equivalent circuit according to Fig. 3 will be explained. In Fig. 3 it was assumed that the internal resistance of the phototransistor 2 is now 4 k52, this value being the switching of the comparator 7 caused the active state. Such a condition is possible, for example by introducing a document with relatively high transparency between Light source and phototransistor are evoked, taking such documents are given for example by tracing paper or folding paper.

It can easily be calculated that there is a voltage in these states of approximately 2.8V is fed to the input 6, while a threshold voltage of approximately 3.1V is fed to the input 8 and the output 9 to approximately 4.85V.

The external load impedance is neglected here. It is clear obvious that a steady state is achieved by conventional circuits cannot be achieved with a fixed threshold value.

It should also be noted that the change in the threshold value in the direction of to a value that constitutes the active stable state has no problem causes the device to switch to idle mode. Indeed it is easy recognizable that it is sufficient to switch to the idle state, the lighting of the photodetector to increase the internal resistance of the phototransistor 2 to a level at which a voltage is slightly greater than 3.1V the signal input 6 is fed. The internal resistance of the phototransistor should therefore be lowered to about 2.45 k52, which value in any case higher than the original resistance value for full lighting.

Therefore, if the document is removed from the position in which it interrupts the light cast on the phototransistor, the device becomes undoubtedly brought back to sleep, with still sufficient lighting changes are permitted before the correct switching behavior is impaired.

Such changes in lighting can be caused, for example, by voltage changes, Dust, etc. are caused.

Let us now consider the case in which a highly opaque Document is brought between the light source and the photodetector, whereby the photodetector practically has an infinitely high resistance. In this Case, the electrical behavior of the circuit can easily be based on the equivalent Circuit according to FIG. 4 are explained. With the assumed conditions lies a voltage of approximately 1.05V at the input 6, a threshold value of approximately 1.6V is formed at input 8, and the voltage level is located at output 9 to 4.75V. Even in this case there is still a voltage difference of about 0.5V between the two inputs, but the effective voltage of the both inputs has been largely shifted to ground, so that a switchover in the reset state even with a slight increase in the lighting of the Phototransistor 2 takes place. Such an increase corresponds to an easily ascertainable one Manner of a decrease in the internal resistance of the phototransistor 2 to 8.5 k52 .

FIGS. 5 and 6 show the level changes over time the input signal, the reference signal and the output signal for two examples considered earlier. Diagram a shows in a solid line the reference value (threshold value). Diagram b shows in a dashed line the value of the input signal. Diagram c shows in a dash-dotted line the output signal.

According to FIG. 5, the circuit arrangement is originally in Idle state corresponding to a full illumination of the phototransistor 2. At the time t1 becomes a relatively transparent document between the light source and the Led phototransistor. The signal level therefore falls below the threshold value, and the comparison circuit switches to the active state. Between the point in time t1 and the point in time t2, a signal transition takes place, which is determined by the time constant of the circuit including the capacitor 15 is defined. During the transition the signal level and the threshold level tend to be exponential against new values. The output signal, which initially had a value of OV, increases with a slight capacitive hysteresis effect to around 4.8V.

The hysteresis effect is caused by the capacitor 15, the changes its charge level.

Once the stabilized active state is reached, so will it is maintained during the entire time interval between times t2 and t3, whereby the document is located between the light source and the photodetector is located.

If the document is removed at time t3, then begins between the times t3 and t4 a new signal transition at which the signal level and the threshold level to the original quiescent value and the output signal instantly returned to the value 0.

The waveform in FIG. 6 essentially corresponds to the waveform according to FIG. 5 and differs only in the Respect that he is related to a case where an opaque document requires activation the circuit arrangement causes.

To complete the description it should be mentioned here that the variable Resistor 14, which is not absolutely necessary, is used to increase the sensitivity of the device and the response behavior of the same for the different Equalize operating conditions.

In the same way divides for the capacitor 16 that this for the Function is not required and is only used to increase the sensitivity of the Circuit arrangement to reduce light changes of relatively high frequency, the for example through the grain size of semi-permeable documents in their Movement between light source and photodector are caused.

Obviously, many changes to the circuitry can be made without departing from the scope of the present invention is left. In particular, it can be seen that the comparator circuit by a differential amplifier or comparison circuit can be replaced at which the resistor 10 forms an integral part.

L e r s e i t e

Claims (6)

Photosensitive detector Claims: 1. Photosensitive Detector for documents, characterized by a light sensor (2) which is connected to a Voltage source (V) is connected and has an internal resistance that changes as a function of the detected radiation; a comparison circuit (7) with a non-inverting input (8), an inverting input (6) and an output (9), a return (11) from the output (9) to the non-inverting Entrance (8); a connection of one of the inputs (6,8) with the Li. ': htsensor (2), to supply a signal voltage; and resistors (12-14) and a first capacitor (15) comprehensive circuit means connected between the inputs and ground are to be a variable reference voltage depending on the signal voltage to be delivered to one of the inputs. 2. Detector according to claim 1, d a d u r c h g e k e n n -z e i c h n e t that the light sensor (2) is connected to the inverting input (6) and that the first capacitor (15) between the non-inverting input (8) and ground is connected. 3. Detector according to claim 2, d a d u r c h g e k e n n -z e i c h n e t that the resistors have a first resistor (12) between the inputs (6,8) and a second resistor (13) between the inverting input (6) and ground include. 4. Detector according to claim 3, d a d u r c h g e k e n n -z e i c h n e t that in addition to the second resistor <13) another variable resistor (14) is connected in series. 5. Detector according to claim 1 or one of the following, g e k e n n z e i c h n e t d u r c h a second capacitor (16) between the inverting Input (Sr and mass. 6. Detector according to claim 5, d a d u r c h g e k e n n -z e i c h n e t that the capacitance of the second capacitor (16) is small in relation to the capacitance of the first capacitor (15).