JPH02119418A - Photodetection circuit for photoelectric switch - Google Patents

Abstract

PURPOSE:To detect an object efficiently by connecting a midpoint of a variable resistor provided between power supplies to a transistor-(TR) of one photodetector in the case of amplifying a photo current caused through the use of two photodetectors at a differential amplifier circuit. CONSTITUTION:As a midpoint voltage is increased, a current flowing to a diode d2 connected in parallel with a transistor(TR) q33 is increased and then a resistive component caused at a tangential line of a voltage current characteristic of the diode is decreased attended therewith, resulting in absorbing a current to the TR q33. Thus, the sensitivity of the photodetector at the side of the TR is decreased and the sensitivity balance of two photodetectors PD1, PD2 is changed. Since the midpoint potential of the variable resistor is given to an output side of a TR q39 of a constant current source of an AC amplifier, as the midpoint level is increased, the gain of the AC amplifier 1 is lowered, and as the midpoint level is close to zero conversely the gain of the AC amplifier is increased. Thus, an object is detected efficiently.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light receiving circuit for processing a photocurrent generated by a light receiving element of a photoelectric switch as a detection signal.

[Conventional technology]

Generally, a photoelectric switch amplifies the photocurrent generated by a photodetector when receiving light of a certain wavelength, and uses a signal above a certain level as a detection signal to determine the presence or absence of an object. Due to the strength of the light source itself, problems arise when the current amplification method is kept constant. In other words, there are summative amplification and differential amplification methods for amplification using active elements, but differential amplification is not suitable when the photocurrent is weak, and conversely, when the photocurrent is strong, it is not suitable. It is preferable to employ differential amplification with higher precision. Therefore, in the photoelectric switch, the electric circuit is configured so that both amplification means can be switched as appropriate.

In addition, an AC amplifier is provided at the next stage of the photocurrent amplification means to amplify the voltage of the signal to the extent that it can be processed by the detection circuit, but it would be more convenient if the gain of this AC amplifier could be changed. Therefore, the amplified signal is controlled by a gain adjustment circuit, but this is done by a circuit separate from the above-mentioned amplification means.

[Problem to be solved by the invention]

In the conventional photodetector circuits mentioned above, each function is processed by a separate circuit, but with recent advances in integrated circuits, photoelectric switches are now integrated with as many circuits as possible to provide reliability and reliability with fewer failures. There is now a need to provide highly functional equipment.

However, if an integrated circuit is to have more functions inside, the number of terminals will increase accordingly, but the shape of the IC chip will also become larger, creating a contradiction in that miniaturization is not possible. The problem is that the longer the wire bonding, the more the number of wire bonding points, the greater the risk of wire breakage due to vibration, and the lower the reliability of the device. However, in the conventional photoelectric switch circuit, a separate circuit is configured for each function, so it is necessary to provide one external terminal for each function.

Furthermore, in the present invention, it is planned to use two light receiving elements, but in this case, two-dimensional detection will be possible by changing the sensitivity balance between the two, and the distance to the detection target will be determined. Therefore, it is preferable to provide a circuit for adjusting the sensitivity balance, but in order to integrate the circuit, the terminal must be shared with the above-mentioned circuit.

The present invention solves the above-mentioned conventional problems, and processes the switching of photocurrent amplification means, the adjustment of the sensitivity balance of the light receiving element, and the gain adjustment of the AC amplifier using one variable resistor, so that multiple It is an object of the present invention to provide a light receiving circuit for a photoelectric switch that is suitable for integration into an integrated circuit and has the following functions.

[Means to solve the problem]

In order to achieve the above object, the present invention includes a variable resistor provided between power supplies, two light receiving elements PDI and PD2, a differential amplifier circuit that amplifies the photocurrent generated by these light receiving elements, and a differential amplifier circuit that amplifies the photocurrent generated by these light receiving elements. It has an AC amplifier that amplifies the signal generated in the amplifier circuit, and connects the midpoint of the variable resistor to the transistor q33 on one of the light receiving elements PDI side of the differential amplifier circuit, and also connects a diode to this one light receiving element. q2 were connected in parallel, and the midpoint of the variable resistor was further connected to the output side of the constant current source transistor q39 of the AC amplifier.

] The voltage at the midpoint of the variable resistor can be varied continuously from Ov to the power supply voltage, but if it is set to Ov, all the elements with switching functions up to transistor q33 on one side of the differential amplifier circuit are completely turned off. becomes the state of So 2
The photocurrents of the two light-receiving elements are equally amplified by respective amplification transistors, and the total current becomes the amplified current. Next, when the midpoint voltage is increased, the potential of the transistor q33 is increased, so that the original function of the differential amplification circuit is exhibited, and the two amplification transistors perform differential amplification.

Also, as the midpoint voltage increases, transistor q33
The value of the current flowing through the diode d2 connected in parallel with the transistor q33 also increases, and the tangential resistance of the diode decreases accordingly, so that the current flowing to the transistor q33 is absorbed. Therefore, the sensitivity of the light receiving element on this transistor side decreases,
The sensitivity balance of the two light receiving elements changes.

Furthermore, since the midpoint potential of the variable resistor is applied to the output side of transistor q39 of the constant current source of the AC amplifier, as the midpoint potential increases, the gain of the AC amplifier decreases, and conversely, the midpoint potential becomes zero. The closer you get, the thicker the gain of the AC amplifier becomes.

〔Example〕

An embodiment of the present invention will be described in more detail with reference to an electric circuit diagram. In FIG. 1, reference numeral 1 denotes a differential amplification type AC amplifier for amplifying the received light signal to a certain extent and then amplifying it to the voltage level of the detection signal; 2 shows the detection signal after amplification. Also,
a is the wiring connecting the light receiving circuit and the variable resistor VR, b-c
is the power line.

Here, the photodiodes PDI and PD2 are divided into two.
are input to a pair of differential amplifier circuits formed by transistors q33 and q34, respectively, but now an external variable resistor V
When the voltage of R is Ov, that is, the middle point of the variable resistor is located on the GND side, the transistors q31, q36, q3
2, q33 and diodes d1, d2 are all in a power-off state and can be ignored in terms of the circuit. On the other hand, transistors q35 and q41 constitute a current mirror, so transistor q35 has transistor Q.
A current of the same magnitude as 41 is about to flow, but if we set the condition that resistor r36 > r3B, it will no longer be possible to flow the specified constant current to transistor q35, so it will become saturated. The state is the same as when it is on, and the impedance of transistor q35 becomes almost zero. Therefore, the photocurrents generated in the photodiodes PDI and PD2 are equally amplified by the transistors q33 and q34, respectively, and the collector currents are both amplified by the diodes d3 and d.
4 is collected in the resistor r36, and as a result, it functions as a summation amplifier circuit which amplifies the output again and then adds the value.

On the other hand, when the voltage applied to the wiring a is increased to the power supply voltage by adjusting the variable resistor VR, the transistor Q31
The emitter potential of transistor q33 and the collector potential of transistor q33 rise, and current is supplied to the current mirror of q35 through transistor q33, so that transistors q33 and q34 operate as a differential amplifier. That is, the base of transistor q33 is on the positive input side, and the base of transistor q34 is on the negative input side. Therefore, when the current value increases, the electromotive current of the photodiode PDI connected in parallel with the diode d2 is absorbed as the tangential resistance of the diode d2 decreases, and its output becomes smaller. FIG. 2 shows changes in the tangential resistance value of the diode d2. Here, the curve is the current-voltage characteristic of the diode d2, and the straight line is the tangential resistance at each current value. In this way, by bringing the voltage of wiring a closer to the electric voltage a by operating the external variable resistor VR, the sensitivity of photodiode Pill can be lowered, so by applying an appropriate voltage, photodiodes PDI and PD2 can be adjusted. This allows for two-dimensional detection in which the detection area is treated as a surface.

Furthermore, as another function, the gain of the AC amplifier 1 at the next stage of the light receiving circuit can be adjusted by changing the variable resistor VR. Since the emitter potential of the transistor q39, which is a constant current source of the AC amplifier 1, is connected to the wiring vAa through the resistor r39, the potential of the wiring a becomes O.
When v, that is, the middle point of variable resistance νR, is on the GND side, it becomes a constant current source for transistor q39, so transistor q33
The base of is maintained at a fixed potential of 2VF (forward voltage of the diode) through the resistor r34, and this becomes the bias point of the differential amplifier circuit. Therefore, it is possible to realize a circuit that adds the outputs of the photodiodes PDI and PD2 until the potential of the wiring a reaches 2VF, and performs a subtraction operation once it exceeds 2VF.

Furthermore, photodiodes PDI and PD2 are normally placed in the same state during differential amplification, but
In actual use, PDI for photodiode PD2
It is necessary to make it possible to lower the sensitivity of the sensor and to make the detectable range variable. This photodiode PDI
, PD2, transistors q42, q43 and diode d2 are used as a configuration for balance adjustment of PD2. In this configuration, when the voltage of the wiring a is gradually increased, the emitter potential of the transistor q43 is also increased, but the constant current flowing through the transistor Q42 is also gradually increased. This current flows through the diode d2, but as the current increases, the tangential resistance ΔR of the diode d2 decreases until the maximum current flows, and when it reaches about 150 mV, it almost stops flowing. Furthermore, since the amplification factor of the differential amplifier circuit in the AC amplifier 1 is proportional to the constant current source of the transistor q39, it is possible to change the gain of the AC amplifier 1 using an external variable resistor VR. Therefore, in this circuit, the gain is variable depending on the DC voltage applied to the wiring a, so it is possible to control the gain using a control voltage from another circuit, or to perform remote control by applying a DC voltage from a distance.

Now, using this circuit, we can convert a photoelectric switch into a one-chip I
If you decide to configure it as C, some part of the distribution
By simply dividing the internal and external circuits of C into circuits, the above-mentioned functions can be shared by the (2) terminals, making it possible to significantly reduce the number of terminals. FIG. 3 shows a block diagram when this circuit is divided by wiring a to form an integrated circuit. Here, 3 is a light receiving circuit which is a main part of the circuit of the present invention, 4 is also an AC amplifier, and CMPI to 3 are comparators.
I and CHF2 determine the stable level and input it as a stable output signal to the stability display circuit 5, and CMP2 determines the operating level and outputs it as a detection signal to the detection circuit 6.
and input to the output circuit 7. Note that the range surrounded by the dotted line is a portion that can be configured as an IIL circuit.

〔Effect of the invention〕

As described above, the present invention uses two light-receiving elements, and when amplifying the photocurrent generated by them with a differential amplifier circuit, the midpoint of the variable resistor provided between the power supplies is connected to the transistor on one side of the light-receiving element. Therefore, when the photocurrent is small with the midpoint voltage, the amplification is switched to summative amplification, and when the photocurrent is large, the amplification is switched to differential amplification, making it possible to efficiently detect an object.

In addition, as the midpoint voltage increases, the sensitivity balance of the two light receiving elements can be changed, making it possible to specify the detection area when the distance is close, making it possible to perform two-dimensional detection. .

Furthermore, since the midpoint potential of the variable resistor is applied to the output side of the constant current source transistor of the AC amplifier, when the midpoint potential is low and the efficiency of summative amplification is low, the gain of the amplifier is increased; When the point potential is high and differential amplification is performed, the gain of the amplifier can be suppressed, making it possible to provide an extremely efficient light receiving circuit for a photoelectric switch.

Furthermore, if a part of the wiring drawn out from the middle point of the variable resistor is used as a terminal and the light receiving circuit side is made into an IC, it is possible to manufacture an integrated circuit having multiple functions with one terminal.
It is possible to provide an IC with high reliability and reduced overall number of terminals.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the light receiving circuit of the photoelectric switch of the present invention, and FIG. 2 is a graph showing changes in the tangential resistance of the diode.
FIG. 3 is a block diagram of essential parts of a photoelectric switch using the circuit of the present invention. In addition, in the figure, 1... AC amplifier, 2... detection signal, 3
... Light receiving circuit, 4... AC amplifier, 5... Stability display circuit, 6... Detection circuit, 7... Output circuit. that's all

Claims (1)

[Claims] 1. Variable resistor VR installed between power supplies and two light receiving elements P
D1 and PD2, a differential amplifier circuit that amplifies the photocurrent generated by these light receiving elements, and an AC amplifier 1 that amplifies the signal generated by this differential amplifier circuit, and the midpoint of the variable resistor and the A transistor q33 on one side of the light receiving element PD1 of the differential amplifier circuit is connected, a diode d2 is connected in parallel to this one light receiving element, and the midpoint of the variable resistor and the constant current source transistor of the AC amplifier are connected. q3
9. A light receiving circuit for a photoelectric switch, characterized in that the output side of the photoelectric switch is connected to the output side of the photoelectric switch.