JP2761702B2 - Photodetector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, such a photodetecting device is configured as shown in FIG. 6, for example. The light modulation type light detection device 1 includes a light emitting element 2 such as an LED that irradiates light to the detected object, and the light emitting element 2 emitted from the light emitting element 2 and reflected on the surface of the detected object or transmitted through a passage or the like of the detected object. And a light receiving IC 3 for detecting light.

The light receiving IC 3 includes a light receiving element 3a such as a photodiode for receiving the 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 an output of the comparator 3c in synchronization with a clock signal of an oscillator 3d.
e and a decision circuit 3f that outputs an H or L level voltage based on the output from the synchronization detection circuit 3e at a predetermined decision 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
Is supplied with a drive voltage from the constant voltage circuit 3h.

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

According to the photodetector 1 configured as described above, the pulse light (see FIG. 7A) emitted from the light emitting element 2 is reflected on the surface of the detection object or passes through the passage of the detection object. After that, the light enters the light receiving element 3 a of the light receiving IC 3.
Thereby, the light receiving element 3a outputs an electric signal modulated by the pulse light corresponding to the intensity of the incident light (FIG. 7).
(See (B)), the output signal from the light receiving element 3a is amplified by the amplifier 3b, and then has a comparator 3c having a hysteresis characteristic, a synchronization detection circuit 3e, and a determination circuit 3f.
And is output as a detection signal via the output buffer circuit 3g (see FIG. 7C).

At this time, when the output signal from the light receiving element 3a exceeds the first threshold value over the entire predetermined judgment time, the output of the judgment circuit 3f becomes H level, and the output terminal 4 is at the L level. When the output signal from the light receiving element 3a becomes equal to or less than the second threshold value lower than the first threshold value over the entire predetermined determination time, the output of the determination circuit 3f becomes , L
Level, and 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.

[0007] Thus, when there is an object to be detected, 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 is reduced. Can be. And this output terminal 4
An appropriate process can be performed by directly inputting the binarized output signal from TTL to, for example, TTL. If the output signal from the light receiving element 3a exceeds the first threshold value during a predetermined determination time due to a logical difference between the output buffer circuits 3g, the output terminal 4
It is also possible to configure so that the output signal from the output terminal 4 goes to L level when the output signal from H goes to H level and falls below the second threshold value.

[0008]

However, in the optical modulation type photodetector 1 having such a configuration, the signal level serving as a reference for detection is larger than the first threshold value when no detection is performed, and At the time of detection, it is necessary to 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, since the variation in performance among products is relatively large, a desired detection accuracy may not be obtained.
Furthermore, if the signal level of the detection reference falls between these two threshold values, there is a problem that detection becomes impossible.

For this reason, the hysteresis characteristic is adjusted by forming the signal processing circuit separately from the light receiving element without using the light receiving IC, but such a configuration increases the number of parts, and There has been a problem that the cost of assembly is increased.

In view of the above, the present invention provides a light receiving IC
It is an object of the present invention to provide a simple and low-cost photodetection device that can reliably detect a signal even when the S / N ratio is small by using the photodetector.

[0011]

In order to achieve the above object, a light modulation type photodetector according to the present invention comprises a light emitting element for irradiating light to a detection object, and a light emitting element for emitting light from the light emitting element. A light-receiving element that receives light transmitted through or reflected by the surface, an amplifier circuit that amplifies a light-receiving signal from the light-receiving element, a comparator that has a hysteresis characteristic that compares an output signal from the amplifier circuit with a reference value, and an oscillator that outputs the output of the comparator. Detection circuit that detects in synchronization with the clock signal of the above, a determination circuit that outputs an H or L voltage based on an output from the synchronization detection circuit at a predetermined determination time, and an output buffer circuit that inverts based on the output voltage of the determination circuit and an optical detection device and an optical modulation type receiving IC comprising a light-emitting element from the pulse drive circuit for pulse emission, than the pulse generation period of the light emitting element A pulse generator for generating a pulse signal of a minute long cycle, current bypass transistor being turned on and off based on the pulse signal of the pulse generator
And a light receiving IC based on the pulse signal from the pulse generator
Output signal from the transistor on pulse rise
Synchronize immediately before, and the signal at this time until the next pulse signal
And an output processing circuit for maintaining and outputting . And
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 light modulation type light detecting device according to the second aspect, the light emitting element is provided at least for a predetermined judgment time of the judgment circuit of the light modulation type light reception IC from the time when the detection signal of the light modulation type light reception IC is output. Turns off only for a while.

[0013]

According to the above arrangement, since the light is turned off for at least a predetermined judgment time in a cycle sufficiently longer than the LED pulse light emission cycle, the signal level of the light receiving element becomes longer than the LED pulse light 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 lower second threshold value that constitute the hysteresis characteristic of the processing circuit,
In a period sufficiently longer than the LED pulse emission period, the hysteresis characteristic becomes equal to or less than the second threshold value. Therefore, even when the signal level of the detection reference is between the first threshold value and the second threshold value, the output signal is surely 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 reliably performed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail based on 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, a light detection device 10 irradiates light to a detection object.
A light emitting element 11 such as an ED, and light emitted from the light emitting element 11;
A light receiving IC 13 for detecting the light reflected on the surface of the detection object or transmitted through a passage or the like of the detection object.

Here, the light receiving IC 13 has the same configuration as the conventional light receiving IC 3 shown in FIG. 6, and is configured as a transmission type in the case shown in FIG. As a result, the output signal becomes L level. If there is an object to be detected, the amount of received light is small, and the output signal is at H level. In this embodiment, the determination circuit 13f performs the determination operation when the same signal is continuously input for two pulses of the light emission of the light emitting element 11, and outputs the H level or L level signal. I do.

In the photodetector 10 described above, the current bypass transistor 12 is connected 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 an output terminal 16 via an 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 photodetector 10 according to the first embodiment is configured as described above, and the pulse light emitted from the light emitting element 11 driven by the pulse signal (see FIG. 2A) After being reflected on the surface or passing through the passage of the detection object, the light enters 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
A pulse signal having a cycle sufficiently longer than the pulse emission cycle of the light emitting element 11 as shown in FIG. As a result, the transistor 12 is turned on when the pulse signal is at the H level, and short-circuits 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.

Thus, when the detected object changes from non-existent to present to non-existent, the output signal of the light-receiving element 13a of the light-receiving IC 13 becomes as shown in FIG.
(D) is obtained. That is, when there is no detected object, the output signal pulse exceeds the first threshold. If there is an object to be detected, the light-emitting element 11 depends on the object.
, The output signal pulse is below the second threshold value when the S / N ratio is large, and can be determined immediately. However, when the S / N ratio is small, the output signal pulse can be determined immediately. It is between the first threshold and the second threshold.

Here, the light receiving IC 13 makes a determination. 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 is turned 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 light exceeding the first threshold is incident for only two light emission pulses. Subsequently, as shown by reference numeral B, while the light emitting element 11 is turned off (see FIG. 2C), the light receiving signal is equal to or lower than the second threshold value (see FIG. 2D). After a lapse of two pulses, the output signal of the light receiving IC 13 is inverted from L level to H level. Again, the code C
As shown by, when the light receiving signal (see FIG. 2D) exceeds the first threshold value and two pulses have elapsed, the light receiving IC 13
Is inverted again from the H level to the L level.

Thereafter, as shown by the reference symbol D, when the detected object is in the state and there is a lapse of two pulses, the light receiving signal falls below the first threshold, but the light receiving signal falls below the second threshold. Since it does not fall below the threshold, the output signal of the light receiving IC 13 remains at the L level. Subsequently, as shown by a symbol E, when two pulses have elapsed, the light emitting element 11 is turned off during this time, and the output signal of the light receiving IC 13 is inverted from the L level to the H level. Further, as shown by the symbol F, 2
Even after the elapse of the pulse, the light receiving signal does not exceed the first threshold value because there is an object to be detected.
Remains at the H level.

As a result, as shown in FIG. 2F, the output signal from the output terminal 16 becomes L when there is no detected object.
When there is an object, the level becomes H level. At that time, a delay time t ₁ from power-on to output determination, and thereafter, a light-off time of the light emitting element 11, that is, a pulse generator 14 The delay times t ₂ and t ₂ are determined based on the pulse length and the determination time by the determination circuit in the light receiving IC 13.
₃ occurs.

Thus, the light receiving signal when the S / N ratio is relatively small and the detected object is present 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 object is reliably detected.

FIG. 3 shows a second embodiment of the optical modulation type photodetector according to the present invention. That is, in FIG. 3, the light modulation type light detection device 20 is the light detection device 10 of FIG.
In response to this, the light emitting element 11 is driven by the drive circuit 21 to emit light, and the drive circuit 21
The drive control is performed via a switch circuit 22 which is turned on only when the light emission pulse No. 3 and the pulse signal from the pulse generator 14 are not input.

The light modulation type photodetector 20 thus configured operates in the same manner as the photodetector 10 of FIG.

FIG. 4 shows a third embodiment of the optical modulation type photodetector according to the present invention. That is, the light modulation type photodetection device 30 triggers the photodetection device 10 of FIG. 1 at the rising (or falling) of the output signal of the light receiving IC 13 instead of the pulse generator 14, A timer circuit (or a one-shot multivibrator circuit) 3 for sending a pulse signal to 12
And a sample-and-hold circuit that synchronizes the output signal of the light receiving IC 13 with the rise and fall of the light emitting pulse of the light emitting element 11 after the determination time of the light receiving IC 13 instead of the output processing circuit 15. An output processing circuit 32 is provided.

According to the light modulation type light detecting device 30 configured as described above, the pulse light emitted from the light emitting element 11 driven by the pulse signal (see FIG. 5A) is reflected on the surface of the detection object. Or through the passage of the object,
The light enters a light receiving element (not shown) of the light receiving IC 13.

When the output signal of the light receiving element 13a exceeds the first threshold level for a time equal to or longer than the judgment time, the output signal of the light receiving IC 13 changes from the H level to the L level, and at the same time, the timer circuit 31 is sufficiently longer than the judgment time. During this time, an H level signal is output. As a result, the base of the current bypass transistor 12 connected in parallel to the light emitting element 11 is provided in FIG.
(B) is input from the timer circuit 31, and the transistor 12 is turned on when this pulse signal is at the H level, thereby short-circuiting 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 changes from non-existence to existence to non-existence, the output signal of the light-receiving element of the light-receiving IC 13 is as shown in FIG.
It becomes as shown in. That is, when there is no detected object,
The output signal pulse exceeds the first threshold. Further, when there is a detection object, the light from the light emitting element 11 is blocked by the detection object.
When the ratio is large, the value becomes equal to or less than the second threshold, and the determination can be made immediately. However, when the S / N ratio is small, the value falls between the first threshold and the second threshold.

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

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

Here, when the detected object is in the state and the time corresponding to two pulses has elapsed, the light receiving signal falls below the first threshold, but does not fall below the second threshold. Therefore, the output signal of the light receiving IC 13 remains at the H level.

When no object is detected, the received signal becomes equal to or higher than the first threshold value. Therefore, when two pulses have elapsed, the output signal is inverted to the L level.

Thus, as shown in FIG. 5B, the synchronization timing of the output processing circuit 32 is set when the pulse signal from the timer circuit 31 rises and after two pulses have elapsed from the fall. Accordingly, the output signal from the output terminal 16 is set to the L level when there is no detected object, and to the H level when there is the detected object, as shown in FIG. 5 (F). At this time, based on a delay time t ₄ from when the power is turned on to when the output determination is performed, and based on the time during which the light emitting element 11 is turned off, that is, the pulse length of the pulse generator 14 and the determination time by the determination circuit in the light receiving IC 13, The delay times t ₅ and t ₆ 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 ,
It is shorter than t ₂ and t ₃ . Thus, more accurate detection of the detected object is performed.

As described above, the light receiving signal when the S / N ratio is relatively small and there is an object is determined by the difference between the first threshold value and the second threshold value of the determination circuit having the hysteresis characteristic. , The presence or absence of the detected object is reliably detected.

[0038]

As described above, according to the present invention, the light emitting element is turned off at least for a predetermined judgment time in a cycle sufficiently longer than the LED pulse light emitting cycle, so that the signal level of the light receiving element is reduced. The signal level of the detection reference is between a first threshold value and a second threshold value lower than the first threshold value, which constitute the hysteresis characteristic of the processing circuit. , It becomes equal to or less than the second threshold value of the hysteresis characteristic in a period sufficiently longer than the LED pulse emission period. Therefore, even when 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 surely inverted at the time of detection, and When an IC is used, detection is reliably performed even if the S / N ratio is small. Thus, according to the present invention, there is provided a photodetection device capable of performing high-accuracy detection by using a light receiving IC even when the S / N ratio is small, so that detection can be performed reliably. Provided.

[Brief description of the drawings]

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

2 (A) to 2 (F) are time charts showing waveforms of respective parts when the photodetector shown in FIG. 1 is operating.

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 a configuration of a third embodiment of the photodetector according to the present invention.

5 (A) to 5 (F) are time charts showing waveforms at various points when the photodetector shown in FIG. 4 is operating.

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

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

(57) [Claims] 1. A light-emitting element for irradiating a detection 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 detection object, and amplifying a light-receiving signal from the light-receiving element. Amplifying circuit, a comparator having a hysteresis characteristic for comparing an output signal from the amplifying circuit with a reference value, a synchronization detecting circuit for detecting an output of the comparator in synchronization with a clock signal of an oscillator, It includes a determination circuit that outputs an H or L voltage based on an output, an output buffer circuit that inverts the voltage based on the output voltage of the determination circuit, and a light modulation type light receiving IC that includes a pulse drive circuit that causes the light emitting element to emit a pulse. In the photodetector, a pulse generator for generating a pulse signal having a period sufficiently longer than the pulse generation period of the light emitting element, and the pulse generator A current bypass transistor being turned on and off based on the pulse signal, the pulse signal from the pulse generator
The output signal from the upper Symbol detector IC was synchronized to the on-pulse rise immediately before the transistor based on No., this and
Output processing that maintains the current signal until the next pulse signal and outputs it
And a circuit, wherein the light emitting element is turned off by the current bypass transistor for at least a determination time of the light modulation type light receiving IC. 2. The photodetector according to claim 1, wherein:
The light detection device is characterized in that the light emitting element is turned off at least during a predetermined judgment time of a judgment circuit of the light modulation type light receiving IC from a point in time when a detection signal of the light modulation type light receiving IC is output. apparatus.