JPH0735614A - Photo detector - Google Patents

Abstract

(57) [Abstract] [Problem] To provide a photodetector that reliably performs detection by using a light-receiving IC even when the S / N ratio is low. A pulse generator 1 for generating a pulse signal having a period sufficiently longer than a pulse generation period of a light emitting element 11 in a photodetector 10 including a light modulation type light receiving IC 13
4 and a current bypass transistor 12 that is turned on or off based on the pulse signal of the pulse generator, and the light emitting element 11 is turned off by the current bypass transistor 12 for at least the determination time of the light modulation type light receiving IC 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a reflection type or transmission type optical sensor, in which a light receiving element, a light receiving signal processing circuit and a light emitting element pulse driving circuit are integrated on one chip and packaged. The present invention relates to a photodetector using a light modulation type light receiving IC.

[0002]

2. Description of the Related Art Conventionally, such a photo-detecting device is constructed, for example, as shown in FIG. The light modulation type photodetector 1 includes a light emitting element 2 such as an LED for irradiating the detection object with light, and the light emitted from the light emitting element 2 and reflected on the surface of the detection object or transmitted through a passage of the detection object. It is composed of a light receiving IC 3 for detecting light.

Here, the light receiving IC 3 includes a light receiving element 3a such as a photodiode for receiving the above light, an amplifier 3b for amplifying a light receiving signal of the light receiving element 3a, and a hysteresis for comparing an output signal from the amplifier 3b with a reference value. A comparator 3c having characteristics and a synchronization detection circuit 3 for detecting the output of the comparator 3c in synchronization with the clock signal of the oscillator 3d.
e and a determination circuit 3f that outputs an H or L level voltage based on the output from the synchronization detection circuit 3e at a predetermined determination time.
And an output buffer circuit 3g. The amplifier 3b, the comparator 3c, the oscillator 3d, the synchronization detection circuit 3e, the determination circuit 3f, and the output buffer circuit 3g.
The drive voltage is supplied from the constant voltage circuit 3h.

Further, the light emitting element 2 is made to emit pulsed light based on a clock signal of the oscillator 3d by a pulse drive circuit 3i provided in the light receiving IC 3.

According to the photodetector 1 thus constructed, the pulsed light emitted from the light emitting element 2 (see FIG. 7A) is reflected on the surface of the object to be detected or transmitted through the path of the object to be detected. After that, the light enters the light receiving element 3a of the light receiving IC 3.
As a result, the light receiving element 3a outputs an electric signal modulated by the pulsed light corresponding to the intensity of the incident light (see FIG. 7).
(See (B)), the output signal from the light receiving element 3a is amplified by the amplifier 3b, and then the comparator 3c having the hysteresis characteristic, the synchronization detection circuit 3e, and the determination circuit 3f.
Is demodulated through the output buffer circuit 3g and output as a detection signal (see FIG. 7C).

At this time, if the output signal from the light receiving element 3a exceeds the first threshold value for the entire predetermined determination time, the output of the determination circuit 3f becomes H level and the output terminal The output signal from 4 becomes L level. When the output signal from the light receiving element 3a becomes equal to or lower than the second threshold value lower than the first threshold value over the entire predetermined determination time, the output of the determination circuit 3f is , L
The output signal from the output terminal 4 is inverted to the H level. At this time, the output signal from the output terminal 4 is stabilized based on the hysteresis characteristic of the comparator 3c.

Thus, when there is a detected substance, the output signal from the output terminal 4 is inverted to the H level because the amount of light incident on the light receiving element 3a decreases, so that the detected substance is detected. You can And this output terminal 4
An appropriate process can be performed by directly inputting the binarized output signal from the device to the TTL or the like. If the output signal from the light receiving element 3a exceeds the first threshold value during the predetermined determination time due to the logical difference of the output buffer circuit 3g, the output terminal 4
It is also possible to configure the output signal from the output terminal 4 to be at the L level when the output signal from is at the H level and is below the second threshold value.

[0008]

However, in the light modulation type photodetector 1 having such a configuration, the signal level serving as a reference for detection is higher than the first threshold value when not detected, and At the time of detection, it must be smaller than the second threshold value. The first threshold value and the second threshold value are determined by the characteristics of the light receiving IC 3. Therefore, there is a case where the desired detection accuracy cannot be obtained because the variation in the performance of each product is relatively large.
Further, when the signal level of the detection reference falls between these two threshold values, there is a problem that detection cannot be performed.

Therefore, the hysteresis characteristic is adjusted by forming the signal processing circuit separately from the light receiving element without using the light receiving IC. However, in such a structure, the number of parts is increased, and There was a problem that the assembly cost would be high.

In view of the above points, the present invention has a light receiving IC.
It is an object of the present invention to provide a simple and low-cost photodetection device that ensures reliable detection even when the S / N ratio is small by utilizing.

[0011]

In order to achieve the above object, a light modulation type photodetector according to a first aspect of the present invention comprises a light emitting element for irradiating a detection object with light and a detection object emitted from the light emitting element. Light receiving element for receiving light transmitted through the surface or reflected by the surface, amplifier circuit for amplifying light receiving signal from the light receiving element, comparator having hysteresis characteristics for comparing output signal from the amplifier circuit with a reference value, oscillator output of the comparator Detection circuit for detecting in synchronism with the clock signal, a determination circuit for outputting H or L voltage based on the output from the synchronization detection circuit at a predetermined determination time, and an output buffer circuit for inverting based on the output voltage of the determination circuit And a light-modulating light-receiving IC including a pulse drive circuit for causing the light-emitting element to emit light in a pulsed manner, and a period sufficiently longer than the pulse generation period of the light-emitting element. It comprises a pulse generator for generating a pulse signal, a current bypass transistor being turned on and off based on the pulse signal of the pulse generator. Then, the light emitting element is turned off by the current bypass transistor for at least the determination time of the light modulation type light receiving IC.

Further, in the optical modulation type photodetector of the second aspect, the light emitting element is at least a predetermined determination time of the determination circuit of the optical modulation type photodetection IC from the time when the detection signal of the optical modulation type photodetection IC is output. Turns off only for a while.

[0013]

According to the above structure, since the light is turned off at least for a predetermined determination time in a cycle that is sufficiently longer than the LED pulse emission cycle, the signal level of the light receiving element is a cycle that is sufficiently longer than the LED pulse emission cycle. Then it becomes almost zero. Therefore, even when the signal level of the detection reference is between the first threshold value and the second threshold value which is lower than that, which constitutes the hysteresis characteristic of the processing circuit,
It becomes less than or equal to the second threshold value of the hysteresis characteristic in a period sufficiently longer than the LED pulse emission period. Therefore, even if the signal level of the detection reference is between the first threshold value and the second threshold value, the output signal will be reliably inverted at the time of detection, and the light receiving IC is used. In this case, even if the S / N ratio is small, the detection is surely performed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in more detail based on the embodiments shown in the drawings. FIG. 1 shows a first embodiment of a light modulation type photodetector according to the present invention. In FIG. 1, the photo-detecting device 10 has an L that irradiates a detected object with light.
A light emitting element 11 such as an ED and light emitted from the light emitting element 11,
The light receiving IC 13 detects the light reflected on the surface of the object to be detected or transmitted through the passage of the object to be detected.

Here, the light receiving IC 13 has the same structure as the conventional light receiving IC 3 shown in FIG. 6, and in the case shown in the figure, it is of a transmissive type and has a large amount of light received when there is no object to be detected. Then, the output signal becomes L level. Further, when there is a detected object, the amount of received light becomes small and the output signal becomes H level. In this embodiment, the determination circuit 13f performs a determination operation on the pulsed light emission of the light emitting element 11 when the same signal is continuously input for two pulses, and outputs a signal of H level or L level. To do.

In the photodetector 10 described above, the current bypass transistor 12 is provided in parallel with the light emitting element 11.
Is connected to the base of the transistor 12,
A pulse signal from the pulse generator 14 is input.

The output signal of the light receiving IC 13 can be output to the outside from the output terminal 16 through the output processing circuit 15 such as a sample hold circuit. The pulse signal from the pulse generator 14 is also input to the output processing circuit 15, so that the output processing circuit 15 synchronizes the output signal from the light receiving IC 13 immediately before the on-pulse of the transistor 12 rises. Has become.

The photodetection device 10 according to the first embodiment is configured as described above, and the pulsed light emitted from the light emitting element 11 driven by the pulse signal (see FIG. 2A) is the detected substance. After being reflected on the surface or transmitted through the passage of the detected substance, it is incident on the light receiving element 13a of the light receiving IC 13.

Here, the base of the current bypass transistor 12 connected in parallel with the light emitting element 11 is shown in FIG.
A pulse signal having a period sufficiently longer than the pulse emission period of the light emitting element 11 as shown in (B) is input from the pulse generator 14. As a result, the transistor 12 is turned on when the pulse signal is at the H level and shorts both ends of the light emitting element 11, so that the light emitting element 11 is turned off. Therefore, the light emitting state of the light emitting element 11 is as shown in FIG.

In this way, when the detected object is changed from nothing to yes to nothing, the output signal of the light receiving element 13a of the light receiving IC 13 is as shown in FIG.
As shown in (D). That is, the output signal pulse exceeds the first threshold in the absence of a detected object. Further, when there is a detected object, the light emitting element 11 is used depending on the detected object.
The output signal pulse is below the second threshold when the S / N ratio is large and can be immediately judged because the light from is blocked, but when the S / N ratio is small, It is between the first threshold and the second threshold.

Here, the light receiving IC 13 makes a determination, and the light receiving IC 13 has a determination time for two light emission pulses of the light emitting element 11. On the other hand, since the pulse generator 14 outputs a pulse signal that is sufficiently longer than the determination time for the two pulses, the light emitting element 11 turns off while the pulse signal is at the H level.

Therefore, when the power is turned on, FIG.
As indicated by the symbol A, the output signal of the light receiving IC 13 is inverted from the H level to the L level when the light exceeding the first threshold value is incident only for two light emission pulses. Subsequently, as indicated by the reference sign B, since the light reception signal is equal to or less than the second threshold value while the light emitting element 11 is off (see FIG. 2C), the light emission is performed. When the time of two pulses has elapsed, the output signal of the light receiving IC 13 is inverted from the L level to the H level. Again, the code C
As shown in FIG. 2, when the light receiving signal (see FIG. 2D) exceeds the first threshold value and two pulses elapse, the light receiving IC 13
The output signal of is inverted from the H level to the L level again.

After that, as indicated by a symbol D, when the detected object is present and the time for two pulses elapses, the light reception signal becomes equal to or lower than the first threshold value, but the second threshold value. Since it does not fall below the threshold value, the output signal of the light receiving IC 13 remains at the L level. Subsequently, as indicated by a symbol E, when two pulses elapse, the light emitting element 11 is turned off during this period, so that the output signal of the light receiving IC 13 is inverted from the L level to the H level. Further, as indicated by the symbol F, 2
Even if the number of pulses elapses, the light receiving signal does not exceed the first threshold because there is an object to be detected.
The output signal of is still at the H level.

As a result, the output signal from the output terminal 16 is L when there is no detected object, as shown in FIG.
The level, or if there is an object to be detected, goes to the H level. At that time, the delay time t ₁ from the power-on to the determination of the output is performed, and then the extinguishing time of the light emitting element 11, that is, the pulse generator 14. Based on the pulse length and the determination time by the determination circuit in the light receiving IC 13, the delay times t ₂ and t
₃ occurs.

Thus, in the case where the S / N ratio is relatively small and the received light signal when there is a detected object is between the first threshold value and the second threshold value of the determination circuit having the hysteresis characteristic. Even if there is, the presence or absence of the detected substance is surely detected.

FIG. 3 shows a second embodiment of the light modulation type photodetector according to the present invention. That is, in FIG. 3, the light modulation type photodetector 20 is the same as the photodetector 10 of FIG.
On the other hand, the light emitting element 11 is driven by the drive circuit 21 to emit light, and the drive circuit 21 causes the light receiving IC 1 to operate.
Driving control is performed via the switch circuit 22 that is turned on only when the light emission pulse of No. 3 and the pulse signal from the pulse generator 14 are not input.

The light modulation type photo-detecting device 20 thus constructed operates similarly to the photo-detecting device 10 of FIG.

FIG. 4 shows a third embodiment of the light modulation type photodetector according to the present invention. That is, the light modulation type photodetector 30 triggers on the rise (or fall) of the output signal of the light receiving IC 13 instead of the pulse generator 14 in the photodetector 10 of FIG. A timer circuit (or one-shot multivibrator circuit) 3 which sends a pulsed signal to 12
1 is provided, and instead of the output processing circuit 15, the output signal of the light receiving IC 13 is synchronized after the determination time of the light receiving IC 13 from the rise and fall of the light emission pulse of the light emitting element 11, such as a sample hold circuit. The output processing circuit 32 is provided.

According to the light modulation type photodetector 30 having such a configuration, the pulsed light emitted from the light emitting element 11 driven by the pulse signal (see FIG. 5A) is reflected on the surface of the object to be detected. Or after passing through the passage of the detected substance,
The light enters the light receiving element (not shown) of the light receiving IC 13.

The output signal of the light receiving element 13a exceeds the first threshold level for the judgment time or longer, and the output signal of the light receiving IC 13 changes from the H level to the L level. At the same time, the timer circuit 31 is sufficiently longer than the judgment time. The signal of H level is output during the period. As a result, the base of the current bypass transistor 12 connected in parallel to the light emitting element 11 is connected to the base of FIG.
A signal as shown in (B) is input from the timer circuit 31, and the transistor 12 is turned on when the pulse signal is at the H level to short-circuit both ends of the light emitting element 11. It is turned off. Therefore, the light emitting state of the light emitting element 11 is as shown in FIG.

Thus, when the detected object is changed from none → present → none, the output signal of the light receiving element of the light receiving IC 13 is as shown in FIG.
As shown in. That is, if there is no detected substance,
The output signal pulse exceeds the first threshold. Further, when there is a detected object, the light from the light emitting element 11 is blocked by the detected object, so the output signal pulse is S / N.
When the ratio is large, it is less than or equal to the second threshold value and can be determined immediately. When the S / N ratio is small, it is between the first threshold value and the second threshold value.

Here, the light receiving IC 13 makes the determination, and the light receiving IC 13 has a determination time for two light emission pulses of the light emitting element 11. On the other hand, since the timer circuit 31 outputs a pulse signal that is sufficiently longer than the determination time for the two pulses, the light emitting element 11 turns off while the pulse signal is at the H level.

Therefore, as shown in FIG. 5 (E), the output signal of the light receiving IC 13 is such that when the power is turned on, the light exceeding the first threshold value is incident for only two light emission pulses. , H level to L level.
Subsequently, since the light receiving signal is equal to or less than the second threshold value while the light emitting element 11 is off, the output signal is inverted from the L level to the H level when the time of two light emitting pulses elapses. . When the light receiving signal again exceeds the first threshold value and two pulses have passed, the output signal of the light receiving IC 13 is inverted from the H level to the L level again. After that, when the light emitting element 11 is turned off, the light receiving signal is equal to or lower than the second threshold value. Therefore, when the time of two light emitting pulses elapses, the output signal is inverted from the L level to the H level.

Here, when the detected object is present and the time for two pulses elapses, the light reception signal becomes equal to or lower than the first threshold value, but not equal to or lower than the second threshold value. Therefore, the output signal of the light receiving IC 13 remains at the H level.

Further, when the detected object disappears, the received signal becomes equal to or higher than the first threshold value, so that the output signal is inverted to the L level when two pulses have elapsed.

Thus, as shown in FIG. 5B, the synchronization timing of the output processing circuit 32 is set at the rising edge of the pulse signal from the timer circuit 31 and after the lapse of two pulses from the falling edge. As a result, the output signal from the output terminal 16 is set to the L level when there is no detected object and the H level when there is a detected object, as shown in FIG. 5 (F). At that time, based on the delay time t ₄ from power-on to output determination, after that, based on the turn-off time of the light emitting element 11, that is, the pulse length of the pulse generator 14 and the determination time by the determination circuit in the light receiving IC 13, Delay times t ₅ and t ₆ will occur. Here, since the synchronization timing of the output processing circuit 15 of the photodetector 10 of FIG. 1 is set only when the pulse signal of the pulse generator 14 rises, the delay times t ₄ , t ₅ ,
t ₆ is the delay time t ₁ in the photodetector 10 of FIG.
It becomes shorter than t ₂ and t ₃ . Thus, more accurate detection of the detected substance is performed.

As described above, the light reception signal when the S / N ratio is relatively small and there is a detected object is between the first threshold value and the second threshold value of the determination circuit having the hysteresis characteristic. Even in the case of, the presence or absence of the detected substance is surely detected.

[0038]

As described above, according to the present invention, since the light emitting element is turned off at least for a predetermined determination time at a period sufficiently longer than the LED pulse light emitting period, the signal level by the light receiving element is reduced. , The signal level of the detection reference is between the first threshold value and the second threshold value lower than the threshold value, which constitutes the hysteresis characteristic of the processing circuit, because the signal level becomes almost zero at a cycle sufficiently longer than the LED pulse emission cycle. Even in the case of, the hysteresis threshold becomes equal to or lower than the second threshold value in a period sufficiently longer than the LED pulse emission period. Therefore, even if the signal level of the detection reference is between the first threshold value and the second threshold value, the output signal of the processing circuit is reliably inverted at the time of detection. When the IC is used, even if the S / N ratio is small, the detection is surely performed. Thus, according to the present invention, even when the S / N ratio is small, by using the light receiving IC, it is possible to perform the detection with high accuracy, which enables the detection with high accuracy. Provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of a photodetector according to the present invention.

FIGS. 2A to 2F are time charts showing waveforms of various parts during operation of the photodetector shown in FIG.

FIG. 3 is a block diagram showing a configuration of a second embodiment of the photodetector according to the present invention.

FIG. 4 is a block diagram showing the configuration of a third embodiment of the photodetector device according to the present invention.

5A to 5F are time charts showing waveforms of various parts during operation of the photodetector shown in FIG.

FIG. 6 is a block diagram showing a photodetector using a conventional light-modulation type light-receiving IC.

7 shows the relationship between the amount of incident light and the time of an output signal in the photodetector of FIG. 6, (A) shows the relationship between the amount of emitted light and time, and (B) shows the relationship between the output signal of a light receiving element and time. , And (C) are graphs showing the relationship between the output signal of the light receiving IC and time.

[Explanation of symbols]

 10, 20, 30 Light modulation type photodetector 11 Light emitting element 12 Current bypass transistor 13 Light receiving IC 14 Pulse generator 15, 32 Output processing circuit 16 Output terminal 21 Drive circuit 22 Switch circuit 31 Timer circuit

Claims (2)

[Claims] 1. A light emitting element for irradiating a detected object with light, a light receiving element for receiving light emitted from the light emitting element and transmitted through or reflected by the surface of the detected object, and a light receiving signal from the light receiving element is amplified. Amplifier circuit, a comparator having a hysteresis characteristic for comparing the output signal from the amplifier circuit with a reference value, a synchronization detection circuit for detecting the output of the comparator in synchronization with the clock signal of the oscillator, and a synchronization detection circuit from the synchronization detection circuit at a predetermined determination time. The light modulation type light receiving IC includes a determination circuit that outputs an H or L voltage based on the output, an output buffer circuit that inverts based on the output voltage of the determination circuit, and a pulse drive circuit that causes the light emitting element to emit light in a pulsed manner. In the photodetector, a pulse generator that generates a pulse signal having a period sufficiently longer than the pulse generation period of the light emitting element, and the pulse generator. A light detection device comprising a current bypass transistor which is turned on or off based on a pulse signal, wherein the light emitting element is turned off by the current bypass transistor at least for a determination time of the light modulation type photodetection IC. apparatus. 2. The photodetector according to claim 1, wherein
Light detection, wherein the light emitting element is turned off for at least a predetermined determination time of a determination circuit of the light modulation type light receiving IC from the time when the detection signal of the light modulation type light receiving IC is output. apparatus.