DE3118838A1 - Photoelectric switching device - Google Patents

Abstract

A photoelectric switching device is specified which has a built-in light source component having an oscillator and a removable light source circuit which is operated by the output signal of the oscillator, and having a removable light source which emits light upon reception of the signal from the light source circuit, a light receiver component having at least one photoelectric transducer for converting the light signal impinging thereon into an electrical signal and a signal processing component for processing the signal based on the converted electrical signal, and an auxiliary component for controlling the mode of operation of the built-in light source component and of the light receiver component, this signal processing circuit having a D flip flop, into which the input data based on the electrical signals are input by transmitting the output signals of the oscillator as signal pulses to the D flip flop via a reversible invertor. In this arrangement, the data signal is read into the D flip flop synchronously with the oscillation of the built-in oscillator (8). The oscillation frequency of the oscillator is locked in by the signal of the impinging light. Since the D flip flop is controlled by clock pulses from the built-in oscillator, undesired parasitic signals due to scattered light and the like are eliminated even when the light source and light receiver are arranged separated at a large distance.

Description

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Photoelectric switching device

description

The invention relates to an electrical switching device, and more particularly to a photoelectric switching device shown in FIG different operating modes can be used.

Photoelectric switching devices are widely used to operate automatic doors or the like. to control, in the conventional photoelectric switching devices a modulated light beam is used to prevent malfunction due to light scattering signals. In the receiver part of the device, the modulated light is detected using a synchronization signal which comes from the light source of the system. Such a synchronized acquisition of the signal becomes what is to be acquired Useful signal optionally formed by removing noise signals. When the light source part and the light receiving part are arranged at a greater distance from each other and both units are operated by different power sources, it is difficult with conventional facilities to transmit the synchronization signal from the light source part to the light receiving part. Therefore, with such Devices with a greater distance between the light source part and the light receiving part ivn instead of synchronization signals measures taken that the pulse signals that are recorded by detecting the 'eighth can be integrated during a predetermined period of time. The output signal is then issued "if

the integrated signal reaches a certain level. At a However, such an arrangement has the disadvantage that it responds relatively slowly, which is due to the use of the Integration circuit is based. The integration circuit can also not recognize the signal if several noise pulses are consecutive can be specified.

An improvement to overcome the shortcomings of the conventional devices has already been proposed. Such a facility is shown in Fig. 1, wherein a light source part A, an oscillator 8, a light source circuit 1 and a Has light source 101. A light receiving part B has a light converter 201, a light receiving circuit 2, a Amplifier 9, a wave shaper 10, a square wave generator 3, an integration circuit 4 and a comparator 11, the output signal of the comparator 11 being given to an output circuit 5. The square wave generator 3 is used to generate a square wave with a significant pulse width every time the square wave generator 3 is triggered by an input pulse with a narrow pulse width, so that the duration can be shortened which is required by the integration circuit in order to generate its output signal of the integration of the input pulseso. However, this device still has the disadvantage that malfunction occurs when one after the other multiple noise signals are input. Therefore it is still necessary to access the receiver part by using Operate synchronization signals.

Previous facilities can be classified into three types. The first type is shown in Fig. 2 (a), in the mode of operation this device the light source part A and the light receiving part B separated from each other and opposite are arranged to each other in order to detect an object M between these components. In this case the light

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beam interrupted by the object M to be detected. The second type is shown in Fig. 2 (b), in this mode of operation the light source part A and the light receiving part B are arranged close to each other and a light reflector D opposite these two parts is arranged to reflect the light from the light source part to the light receiving part. The light path is in turn interrupted by the object M or M 'to be detected. The third type is shown in Fig. 2 (c), wherein in this mode of operation the light source part Λ and the light receiving part B are arranged close to each other and both on the path of the object M to be detected are directed. In this case, the light from the object to be detected M itself reflected.

A comparison of the three different types of photoelectric switching devices gives the following. In the former case the light source part A is arranged in a separate housing from the light receiving part B, so that the detection of the received signal takes place unsynchronized, while in the last two types of device the light source part A in the same housing as the light receiving part B are arranged, so that a synchronization when detecting the signals is possible. In the devices shown in Figs. 2 (a) and 2 (b) the interruption of the light beam is detected to generate an output signal. In this operating mode, in order to to prevent incorrect output signals during the initial transition period immediately after switching on the device. the integration circuit 4 can be operated in a special manner, for example so that a special circuit is provided to the timer capacitor in the integration circuit 4 can be charged quickly during the reception phase. In contrast, the one shown in Fig. 2 (c) Device which detects light reflection to produce an output signal to create. With this device, faulty output signals must be avoided at the initial transition period immediately

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after turning on the device to prevent the integration circuit 4 can be operated in the opposite way, i.e. a special circuit must be provided, to quickly discharge the timer capacitor in the integration circuit 4 during the initial transition phase. Therefore, the devices of the type shown in Figs. 2 (a), 2 (b) and 2 (c) have hitherto had to be manufactured separately, although they have almost the same circuit because they are different Special circuits in the integration circuit 4 were required.

In contrast, the invention is based on the object of specifying an improved photoelectric switching device which switches stably and reliably in each of the three modes mentioned above.

For this purpose, the device according to the invention is described in claim 1 indicated manner, while the subclaims characterize advantageous embodiments of the invention.

The device according to the invention switches itself reliably when used in the manner shown in Fig. 2 (a) with no synchronization signal from the light receiving part B is received by the light source part A while the light from the object M to be detected is interrupted. Therefore, the device according to the invention has great compatibility in various types of use, and is very useful.

From the accompanying drawings show:

Fig. 1 is a block diagram of a conventional photoelectric switching device;

Fig. 2 (a) is a schematic representation of the use of a photoelectric switching device in which the Light source and the light receiver separated from each other

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which are arranged;

Fig. 2 (b) is a schematic representation of the use of the photoelectric switching device, wherein the light source and the light receiver are arranged next to one another and the light beam is interrupted by an object to be detected like a light barrier;

Fig. 2 (c) is a schematic representation of the use of the photoelectric switching device, wherein light source and light receivers are arranged next to one another and the light beam is reflected by the object to be detected will;

Fig. 3 is a block diagram of a photoelectric switching device according to an embodiment of the invention;

Fig. 4 is a circuit diagram of the device of Fig. 3;

5 shows a diagram of the waveforms in their time sequence “Which appear in the various parts of the circuit shown in Figures 3 and 4;

6 is an example of a circuit diagram of the circuit diagram shown in FIGS Figs. 3 and 4 control circuit shown;

7 shows an exemplary embodiment for the one in FIGS. 3 and 4 built-in oscillator shown; and

8 shows a diagram of the waveforms in their chronological sequence in different parts of the circuit shown in FIG.

The photoelectric switching device according to the embodiment of the invention, as shown in FIG. 3, has a light receiving part B, a built-in light source part A, an auxiliary part C and optionally a separate ^ light source part A 'on.

The light receiving part B has a light receiving circuit 2 which takes out the alternating current component of the electric signal produced by converting the light signal incident on a light detector 201, and a signal processing circuit E for processing the output signal of the light receiving circuit 2. The signal processing circuit E has an amplifier 9, a wave shaper 10, a D flip-flop 300, an integration circuit 4 and a comparator 11 connected in series. The comparator 11 emits its output signal to an output circuit 5. The built-in light source part A has a built-in oscillator 8 and a first light source circuit 1, the output signal of which is given to a first light source 101. The separate light source part A ¹ has a separate or second oscillator 8 'and a second light source circuit 1 ¹ , the output signal of which is given to a light source 101'. The auxiliary part C has a control circuit 13, a voltage detector 14, a rapidly chargeable charge / discharge circuit and a switchable inverter 12. The output of the comparator 11 passes through an output circuit 5 to an output terminal 51.

The device shown in Fig. 3 can be used in any of the three forms, shown in Figs. 2 (a) to 2 (c) can be used. For the form of use shown in Fig. 2 (a) As shown, the first light source circuit 1 and the first light source 101 are made up of the built-in light source part A. introduced into the separate light source part A '. For the second and third types of use shown in Fig. 2 (b) and 2 (c), respectively, the first light source circuit 1 and the first light source 101 become in the built-in light source part A used. The various modes of operation of the circuit are selected by the nature of the input signals that can be applied to the control circuit 13.

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The light receiving circuit 2 in the light receiving part B is an AC amplifier that amplifies the AC component of the signal from the photodetector 201 and sent to the amplifier 9 passes on. A wave shaper 10 receives the output signal of the amplifier 9 and gives it at its output pending square wave to a D flip-flop 300. The D flip-flop 300 also receives trigger signals from the built-in oscillator 8 at its clock pulse input ck through the switchable Inverter 12, which is optionally switchable to the oscillator output signals non-inverted or inverted to be passed on, depending on the input control signal which is output by the control circuit 13. The D flip-flop 300 is a flip-flop, which is switched by pulse edges and emits output signals

(a) on a falling edge of the clock pulse when the light detector detects the light from the built-in light source part A receives or

(b) on each rising edge of the clock pulse if the Light detector that receives light from the separate light receiving part A '. The integration circuit 4 is a Low pass filter with a very long time constant and essentially integrates the signal from the D flip-flop 300 and outputs an output signal to the comparator. The comparator 11 outputs an output signal when the integrated output voltage of the integration circuit 4 reaches a predetermined level.

Between the built-in light source part A, the oscillator 8 sends output signals to the first light source circuit 1 in addition to the clock signals at the input ck of the D flip-flop 300, so that the emitted from the light source 101 Light by the frequency of the oscillator signal d_-s oscillator 8 is modulated. The first light source circuit 1 is detachably arranged, and if the device in the first Operating mode (Fig. 2 (a)) is used, the first light

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source circuit plugged into the separate light source part A ', so that they are there as a light source circuit 1 'with the Light source 101 'can be used.

The separate light source part A ¹ and the separate oscillator 8 'are designed so that their oscillation frequency is somewhat (for example 3 to 20%) higher than the oscillation frequency of the built-in oscillator 8. This selection of the ratio between the oscillation frequencies, if the separate oscillator 8 ^{1 is} used, is synchronized by the synchronization loop syc from the wave shaper 10 to the built-in oscillator 8, the frequency to that of the separate oscillator 8 ', so that undesired interference by interference -Light signals is eliminated.

Fig. 4 shows in detail the structure of the oscillator 8, the switchable inverter 12, the charge / discharge circuit 15 and of the voltage detector 14. Fig. 5 is an illustration of the timing of the waveforms in various parts of the circuits shown in Figs.

The built-in oscillator 8 has a function amplifier 16 and oscillator transistors Qg and Qq as oscillator elements, a switching transistor Tr .. to change the time constant of the oscillator 8, and a switching transistor Tr_ to switch the transistor Tr ₁ through the signal from the control circuit 13 turn off (the transistor Tr ₁ is kept in the off state). The switchable inverter 12 has transistors Tr ₃ -Tr ₆ , Tr ₄ -Tr ₇ and Tr ₅ . The base connections of the transistors Tr 1 and Tr_ receive the control signal from the control circuit 13. The output signal of the inverter 12 is taken from the collector connections and applied to the clock input connection ck of the D flip-flop 300.

When the photoelectric converter 201 does not receive a signal from the

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receives a separate light source part A ¹ , the level detector 10 does not emit an output pulse, as can be seen on the right half of the waveform (2) in FIG. In this case, a voltage is present at the upper end of the capacitor C in the oscillator 8, the course of which is a simple charge-discharge curve, which is shown on the right-hand part of the waveform (3). The waveforms of the output pulses of the oscillator 8 are shown at (4), while the output signals of the inverter and the D flip-flop 300 are shown on the right part of the waveforms (5) and (6). In this case, the oscillation frequency of the built-in oscillator 8 is not drawn to the frequency of the separate oscillator 8 ', and the oscillator 8 oscillates at its own frequency, for example 2 kHz.

When the photoelectric converter 201 receives a light signal from the separate light source part A ¹ , the level detector 10 emits output pulses which have the frequency of the separate oscillator 8 ″, for example 2.8 kHz, as shown in the left part of the waveform (2) of FIG 5. In this case, the transistor Tr _{1 is turned on} by the pulse signal from the level detector 10. Therefore, during the turn-on period, a series connection of the internal resistance of the transistor Tr. And the resistor R _{1 is connected in} parallel with the charging resistor R ", so that As a result, the voltage at the upper end of the capacitor C becomes the waveform shown at (3) on the lirx half of Fig. 5. By this approximation of the waveform of the voltage across the capacitor C against the waveform in the state , in which there is no light in the signal M and which is shown on the right half of Fig. 5, becomes the oscillation frequency of the oscillator 8 forcibly brought to the oscillation frequency (for example 2.8 kHz) of the oscillator 8 '. Also the leading edge or the rise time of the output pulses of the oscillator 8

shift slightly forward in phase during the time the light signal is received. Hence this is Output signal of the D flip-flop 300, which is selected so that it is triggered on the falling edge, a Output signal at the high level as shown by waveform (b) on the left of FIG. Around To obtain the mode of operation described in connection with FIG it is required that the oscillation frequency of the built-in oscillator 8 is a little deeper than that of the separate oscillator 8 ", because the forced setting the oscillation frequency of the built-in oscillator 8 to the Oscillation frequency of the separate oscillator 8 'it requires that the mentioned relationship exists between the frequencies, i.e. that the oscillator that is forcibly connected has a has a slightly lower frequency. Experiments show that the differences in frequencies are preferably between 3 to 20% lie. If the frequency difference is less than 3%, The interaction of the oscillators can be due to changes in the working voltages or working temperatures turn into the opposite. With frequency differences of more than 20% it is difficult to keep the frequency entrainment stable to keep.

The transistor Tr ₂ has its collector connected to a diode D and through the diode D is connected to the base of the switching transistor Tr ... The base <is connected to the control circuit 13.

transistor Tr .. connected. The base of the transistor Tr ^ is

When a signal with a high level is applied from the control circuit 13 to the base of the transistor Tr ₂ , the transistor Tr. Is kept in the switched-off state, so that the frequency pickup by the output signal of the level detector does not take place. At the same time, the switchable inverter 12 by receiving a high level output signal from the control circuit 13 to the

non-inverting mode switched so that the output signal of the oscillator 8 is passed on to the clock pulse input ck without inversion.

The voltage detector circuit 14 (Fig. 4) has a constant voltage diode (Zener diode) ZD and a transistor Tr ₁₁ and generates an output signal when the voltage V der

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Power source reaches a predetermined, stable voltage. The output of the constant voltage circuit 14 is sent to the Output circuit 5 passed, which is designed as a gate circuit and prevents the output signal of the comparator 11 is passed to the output terminal 51, so that a malfunction in the initial phase at power-up the facility is eliminated.

The charge / discharge circuit 15 optionally serves to quickly charge or discharge the charge of the capacitor in the integration circuit 4. The circuit 15 has transistors Tr-J ₂ ? ^Tr i4f ^Tr i6 ^{and Tr} 15 ^{and a} diode D ^. This circuit receives control signals Q ₁ , Q- from the control circuit 13 “If the control signals Q ₁ , Q _{1 are} at a high level, the circuit 15 works in the sense of a fast charging. When the control signals Q ₁ , Q _{1 are} at a low level, the circuit 15 operates in the sense of a rapid discharge. The former case is for the modes of use of the device as indicated in Fig. 2 (a), and the latter case is for the modes of use shown in Figs. 2 (b) and 2 (c).

The control circuit 13 serves to generate control signals which are output to the switchable inverter 12, the oscillator 8 and the charging / discharging circuit 15. The control circuit 13 is constructed as shown in FIG. The control circuit has three transistors Tr ₄₁ , Tr ₄₂ and Tr ₄₃ as well as several diodes and resistors. At the collectors

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of the transistors Tr ₄₂ ^and Tr ₄ -, the control signals Q ₁ and Q ₁ for the charge / discharge circuit 15 are available. The control signal Q ₂ for the oscillator 8 and the inverter 12 is applied to the collector of the transistor Tr _41. The output signals of this control circuit are changed depending on the three states at the input terminal S. The operation of the circuit of Fig. 6 will now be described for three cases:

(i) When the input terminal S is at a high level, both transistors Tr ₄₂ and Tr _{43 are} turned on, and the transistor Tr ₄₁ is turned off. Therefore, the output control signals Q ₁ , Q ₁ , Q “are at a low level.

(ii) When the input terminal S is left open, both transistors Tr ₄₂ and Tr _{43 are} turned on so that the output control signals Q., Q ... go to a low level and the output control signal Q ₂ to a high level goes.

(iii) When the input terminal S is brought to a low level, both transistors Tr _ and Tr are turned off, so that the output control signals Q ₁ , Q _{1 are brought} to a high level. Furthermore, the transistor Tr _{41 is} turned on, so that the output control signal Qy comes to a high level.

With respect to the state at the input terminal S. Is the whole Operation of the oscillator 8, the inverter 12 and the charge / discharge circuit 15 as follows:

(i) When the input terminal S is set to a high level:

Both control signals Q ₁ and Q ₂ go to a low level. Since the output control signal Q _{2 is} at a low level, the oscillator 8 is switched to the operating mode in which the oscillation frequency of the separate oscillator

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tors 8 ° is taken, with the oscillator 8 the output signal of the wave shaper 10 receives. The inverter inverts the output signal of the oscillator 8 and passes it on to the D flip-flop 300 as a clock pulse.

Since the output control signal Q _{1 is} at a low level, the charge / discharge circuit 15 functions to charge the capacitor of the integration circuit 4 quickly in the initial stage after the device power source is turned on, so that the device functions as a photoelectric switch can work with separate arrangement of light source part and light receiving part »

(ii) When the input terminal S is open, when the input terminal S is open, the control signal Q n is at a low level and the control signal Q _{2 is} at a high level. When the control signal Q _{1 is} at a low level, the capacitor of the integration circuit 12 is quickly charged in the initial stage after the power source is turned on.

When the control signal Q _{2 is} at a high level, the oscillator 8 is switched to the mode in which there is no entrainment at the oscillation frequency of the separate oscillator 8 ', and the device operates SOp that the inputted modulated signal is detected by the synchronized detector function , the frequency of the built-in oscillator 8 being used. Further, when the control signal Q _{2 is} at a high level, the inverter 12 stops the inverter operation and outputs the oscillation signal of the oscillator 8 to the D flip-flop 300 without inversion.

Hence the facility works shown in Fig. 2 (b).

(iii) When the input terminal S is set at a low level is, the following applies:

Both control signals Q ₁ and Q ₂ go to a high level. When the control signal Q _{1 is} at a high level, the charge / discharge circuit 15 causes the capacitor of the integration circuit to be rapidly discharged in the initial stage after the power source is turned on, so that the device is set to operate in the in the mode of operation shown in Fig. 2 (c) operates ...

When the control signal Q2 is high, will the oscillator 8 and the inverter 12 are adjusted to achieve the synchronized detector mode will.

In this case, therefore, the device operates in the mode shown in Fig. 2 (c).

The following applies to the charge / discharge circuit 15:

(a) When the control signal Q _{1 is} at a low level and when the constant voltage circuit 14 outputs a signal at a high level, the capacitor in the integration circuit is turned off by the high level signal through the transistors Tr ₁₄ and Tr ₁ . ·> Quickly charged because the transistor Tr. _{R is} switched off.

(b) When the control signal Q _{1 is} at a high level _r and when the constant voltage circuit 14 outputs a signal at a high level, the charge on the capacitor in the integration circuit is quickly discharged _{through the transistors Tr 1} r _{and Tr 1, -} , since the transistor Tr., is switched off. As a result of the above-described mode of operation of the circuits 14, 15, 4 and 5, undesired output signals are produced in the output circuit.

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nals or error signals are suppressed until the Circuit 14 detected voltage becomes stable.

The main parts of the circuit described above are manufactured as an IC (Integrated Circuit). The IC has terminals for connecting the first light source part 1, the light receiving circuit 2, the output circuit 5, the positive and negative power source terminals _{f of} the control signal input terminal S and associated components.

Fig. 7 shows another example of the built-in oscillator 8, an output signal being generated on wave shaper 10 by rapidly discharging timer capacitor C the synchronization of this oscillator 8 to the output signal of the wave shaper 10 is carried out. FIG. 8 shows the waveforms in various parts of the circuit of FIG. 7.

If there is no light input signal, the voltage has at the upper end of the timing capacitor C of Fig. 7 has the waveform shown in Fig. 8 (a) so that as the output of the wave shaper 10, the waveform shown in Fig. 8 (b) is generated.

If there is a light input signal, in The output of the wave shaper 10 shown in Fig. 8 (c) shows a rapid discharge of the timer capacitor C of Fig. 7. Therefore the voltage at the upper end of the timer capacitor C has the waveform shown in Fig. 8 (d), so that the output signal of the built-in oscillator, i.e., the output of the operational amplifier 16, that shown in Fig. 8 (e) Waveform received. In this case, the switchable inverter be carried out identically as described above.

Since the D-flip-flop 300 is switched so that it receives a data input based on the light signal and clock signals based on the oscillations of the built-in oscillator 8, the D-flip-flop 300 reads the data signal synchronously with the device according to the invention the oscillation of the built-in oscillator 8, the frequency of these oscillations being carried along with that of the input light signal. Even in the mode of operation shown in FIG Oscillator 8 _f so that undesired interference from stray light is eliminated.

The device according to the invention is for all three modes of operation those shown in Figs. 2 (a), 2 (b) and 2 (c) can be used are by selecting the signal at the input terminal S accordingly. Furthermore, there will be a malfunction of the device prevented in the initial stage after switching on the power source, regardless of the choice of one of the three modes of operation mentioned. This is done by the charge / discharge circuit reached, which is also controlled by the signal at the input terminal S.

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Claims (7)

Expectations Κ / Photoelectric switching device, marked by (a) a built-in light source part (A) with an oscillator (8) and a removable light source circuit (1), which is operated by the output signal of the oscillator (8), and with a removable light source (101), c .e at Reception of the signal from the light source circuit (1) emits light, Cb) a light receiving part (B) with at least one photoelectric Converter (201) to convert the incident light signal into an electrical signal, and a Signal processing part (E), in order to convert the electrical signal-based signal to be processed. th, BATH ORIGINAL (C) an auxiliary part (C) for controlling the operation of the built-in light source part (A) and the light receiving part (B), where (d) the signal processing circuit has a D flip-flop (300) into which the input data is based on the electrical Signals are input by the fact that the output signals of the oscillator (8) via a switchable Inverter (12) are given as clock signals to the D flip-flop (300). 2. Device according to claim 1, further characterized by (e) a separate light source part (A ') with a second oscillator (8'), a removable light source circuit (1 ") and a detachable light source (101 ') to emit light when receiving a signal from the light source circuit (1 ') is received, wherein the detachable light source circuit and the detachable light source are the same components, which can alternatively be connected either in the built-in light source part or in the separate light source part, whereby (f) the second oscillator (8 ¹ ) in the separate light source part (A ¹ ) has a slightly lower oscillation frequency, and wherein (g) the first oscillator (8) in the built-in light source part (A) receives a synchronization signal to stop its operation to synchronize with the converted electrical signal. 3. Device according to claim 2, characterized in that the auxiliary part (C) has a switchable inverter (12), which selectively switches the polarity of the clock pulses applied to the D flip-flop (300), so that the the time sequence of the reading process is switched over by the D flip-flop (300), with an output being output, 118838 either (i) at each fall of the clock pulse when the light receiving transducer receives the light emitted from the built-in light source part (A), or (ii) on each rising edge of the clock pulse when the light receiving transducer receives ^{the light from the separate light source part (A 1).} 4. Device according to claim 1 or 2, characterized in that that the main parts of the circuit are designed as IC components. 5. Device according to claim 3, characterized in that that the auxiliary part (C) has a control circuit (13) with a Control signal terminal has to control the operation of the IC components by outputting a selection signal at the control signal terminal to switch. 6 "Device according to claim 1, further characterized by: (h) a voltage detector circuit (14) which determines whether the voltage of a direct current source for supplying the electrical switching of at least the light receiving part from an initial transitional state after switching on the power source has changed to a steady state, (i) a gate circuit (5) to deliver the output signal of the signal processing part (E) to the output terminal (51) depending on the output of the voltage detector circuit (14) to control, the controller such it is made that the delivery of the output signal is blocked until the steady state has occurred " (j) an integration circuit (4) with a timer capacitor to get the output of the signal processing circuit to integrate, and through BATH ORIGINAL (k) a charge / discharge circuit (15) to the timer capacitor of the integration circuit (4) to charge or to discharge quickly during the period of the transient state. 7. Device according to claim 6, characterized in that that the charge / discharge circuit (15) in its mode of operation a control signal of the auxiliary part (C) is switched over so that it adopts the following operating modes: (i) for the application of the device with separate arrangement of light source and light receiver (Fig. 2a) and for the Use of the device with a combined light source and light receiver and reflector (Fig. 2b) rapid charging is performed by the charge / discharge circuit, and (ii) when using the combined arrangement device the light source and the light receiver and reflection of the object to be detected (FIG. 2c), the charge / discharge circuit (15) performs a rapid discharge. BATH ORIGINAL