JPH0835882A - Pyroelectric infrared sensor circuit and infrared detection circuit - Google Patents

Abstract

PURPOSE:To make raising the gain and the shortening of startup time. CONSTITUTION:The drain D of the first FET 11a and the drain D of the second FET 11b are connected to common constant current circuit 12, which constitutes a differential amplifier with input terminal of each gate G. Then, the signal from a pyroelectric infrared sensor S1 is input in the gate G of the first FET 11a, the carrier wave signal from a carrier wave generation circuit 13 is input in the gate 2 of the second FET 11b and the detection signal Vs is taken out of the source S of the first FET 11a. This detection signal Vs is amplified by a reverse amplifier circuit 20 including a coupling capacitor C1, an input resistor R21, an operation amplifier OP1 and a feedback resistor R22. It is especially useful in opening an automatic door, and human body detection for security in a building.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pyroelectric infrared sensor circuit and an infrared detecting circuit, and more particularly to a pyroelectric infrared sensor circuit and an infrared ray which can achieve both high gain and short start-up time. Regarding the detection circuit.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing an example of a conventional infrared detecting circuit. The infrared detection circuit 500 includes a pyroelectric infrared sensor circuit 50 and an amplifier circuit 60. In the pyroelectric infrared sensor circuit 50,
One end of the pyroelectric infrared sensor S1 has a gate G of the FET 51.
, And the other end is grounded. In addition, the FET
The gate G of 51 is grounded via a gate resistor RG. Further, the drain D of the FET 51 has a power source V
connected to cc. Further, the source S of the FET 51 is grounded via the source resistance RS. The amplifier circuit 60 is an inverting amplifier circuit including a coupling capacitor C1, an input resistor R61, an operational amplifier OP1 and a feedback resistor R62. The reference voltage VREF3 is applied to the non-inverting input terminal of the operational amplifier OP1.

Next, the operation of the above infrared detecting circuit 500 will be described. At power-on, coupling capacitor C
According to the voltage difference between both ends of 1, the coupling capacitor C
1 is charged. During this charging, the operational amplifier output voltage V
Infrared cannot be detected because o'is in a state of being completely swung up to the power supply voltage. That is, it takes some startup time. Since charging is performed through the resistors R61 and R62, the time constant τ = C1 (R61 + R62)
Is a measure of startup time. After the start-up time, if there is no change in infrared, pyroelectric infrared sensor S
Since no charge is discharged from 1, only the bias current flows as the source current Is, and the detection signal Vs ′ becomes a predetermined DC bias voltage. This DC bias voltage is generally about 2V when the power supply Vcc is 5V. Since the DC bias voltage is cut by the coupling capacitor C1, no signal is input to the inverting input terminal of the operational amplifier OP1. Therefore, the operational amplifier output voltage Vo '
Matches the reference voltage VREF3. When the infrared rays fluctuate, the pyroelectric infrared sensor S1 emits electric charges.
Since this charge changes the potential of the gate G, the source current Is changes. As a result, the detection signal Vs' changes. The change range of the detection signal Vs ′ is generally several mV.
Is. The amount of change in the detection signal Vs ′ passes through the coupling capacitor C1 and the resistor R61 and the operational amplifier OP1.
It is input to the inverting input terminal of. Therefore, the operational amplifier output voltage Vo ′ changes in the voltage range obtained by amplifying the change range of the detection signal Vs ′. Feedback resistance R62 / Input resistance R
61 is a measure of the gain of the operational amplifier OP1.

[0004]

As the coupling capacitor C1, a ceramic capacitor or a polymer film capacitor with a small leakage current is used. Therefore, the value of the coupling capacitor C1 is about 10 μF. The low cutoff frequency fz of the amplifier circuit 60 is determined by fz = 1 / (2π · C1 · R61). For human body detection applications, the frequency band of the signal is 1
Hz to 100 Hz, the low cutoff frequency fz
Should be 0.1 Hz or less. Therefore, from the above equation, the input resistance R61 needs to be 160 kΩ or more. On the other hand, since the change range of the detection signal Vo ′ is several mV, in order to set the change range of the operational amplifier output voltage Vo ′ to several V, the gain determined by the feedback resistor R62 / the input resistor R61 needs to be 1000 or more. . Therefore, the feedback resistor R62 needs to be 160 MΩ or more. However, in this case, the startup time determined by the time constant τ = C1 (R61 + R62) becomes about 1600 seconds, which is an unacceptably long time. In order to reduce the startup time to about 160 seconds, the feedback resistor R62 must be set to about 16 MΩ. However, if this is done, the gain will be about 100, and the change range of the operational amplifier output voltage Vo ′ will be several 100 mV.

As described above, the conventional infrared detection circuit 50
At 0, there is a problem that it is not possible to achieve both high gain and shortened startup time. Therefore, an object of the present invention is to provide a pyroelectric infrared sensor circuit and an infrared detection circuit that can achieve both high gain and short startup time.

[0006]

SUMMARY OF THE INVENTION In a first aspect, the present invention is a differential amplifier in which the drain of a first FET and the drain of a second FET are connected to a common constant current circuit and each gate serves as an input terminal. And a signal from a pyroelectric infrared sensor is input to the gate of the first FET, a carrier signal is input to the gate of the second FET, and a detection signal is extracted from the source of the first FET. An electric infrared sensor circuit is provided.

According to a second aspect, the present invention comprises a pyroelectric infrared sensor circuit having the above structure and an amplifier circuit for amplifying a detection signal from the pyroelectric infrared sensor circuit.
The amplifying circuit is an inverting amplifying circuit including a coupling capacitor, an input resistor, an operational amplifier, and a feedback resistor.

[0008]

In the pyroelectric infrared sensor circuit of the first aspect, the drain of the first FET and the drain of the second FET are connected to a common constant current circuit, and a differential amplifier using the respective gates as input terminals is constructed. To do. Then, the signal from the pyroelectric infrared sensor is input to the gate of the first FET,
The carrier signal is input to the gate of the second FET, and the detection signal is extracted from the source of the first FET. Therefore, the detection signal is a signal obtained by amplitude-modulating the carrier signal with the signal from the pyroelectric infrared sensor, and has a frequency much higher than the frequency of the signal from the pyroelectric infrared sensor. Therefore, when the detection signal is amplified by the inverting amplifier circuit including the coupling capacitor, the input resistance, the operational amplifier, and the feedback resistance, the low cutoff frequency fz may be a relatively high frequency, and the value of the input resistance can be reduced. I can. Then, even if the feedback resistance is reduced, a high gain can be obtained. On the other hand, if the input resistance and the feedback resistance are reduced, the start-up time also becomes shorter. Thus, when an amplifier circuit is connected in the subsequent stage, both high gain and short start-up time can be achieved.

In the infrared detecting circuit according to the second aspect, the pyroelectric infrared sensor circuit according to the first aspect is used, and the infrared detecting circuit includes an inverting amplifier circuit including a coupling capacitor, an input resistor, an operational amplifier and a feedback resistor. The detection signal from the electric infrared sensor circuit is amplified. Therefore, similarly to the above, it is possible to achieve both high gain and short startup time.

[0010]

EXAMPLES The present invention will be described in more detail with reference to the examples shown in the drawings. The present invention is not limited to this.

FIG. 1 is a circuit diagram showing an embodiment of an infrared detecting circuit of the present invention. This infrared detection circuit 100 is
It is composed of a pyroelectric infrared sensor circuit 10 and an amplifier circuit 20. In the pyroelectric infrared sensor circuit 10, the drain D of the first FET 11a and the second FET 11b
And the drain D of the same are connected to a common constant current circuit 12, and a differential amplifier using the respective gates as input terminals is configured. One end of the pyroelectric infrared sensor S1 has the first FE
It is connected to the gate G of T11a. Also, the first
The gate G of the FET 11a is connected via a gate resistor RG
It is grounded. The source S of the first FET 11a is grounded via the source resistance RS1. One end of the carrier wave generation circuit 13 is connected to the gate G of the second FET 11b. The source S of the second FET 11b is grounded via the source resistance RS2. The amplifier circuit 20 includes a coupling capacitor C1.
Input resistance R21, operational amplifier OP1, feedback resistance R22
It is an inverting amplifier circuit including and. The reference voltage VREF3 is applied to the non-inverting input terminal of the operational amplifier OP1.

Next, the operation of the above infrared detecting circuit 100 will be described. At power-on, coupling capacitor C
According to the voltage difference between both ends of 1, the coupling capacitor C
1 is charged. During this charging, the operational amplifier output voltage V
Infrared rays cannot be detected because o has been swung up to the power supply voltage. That is, it takes some startup time. Since the charging is performed through the resistors R21 and R22, the time constant τ = C1 (R21 + R22) is a standard for the start-up time. After the start-up time, if there is no change in infrared, pyroelectric infrared sensor S1
Since no electric charge is discharged from the device, only the bias current flows as the source current Is1. However, the second FET1
Since the drain current of 1b changes with the carrier wave, the bias current also changes with the carrier wave. Therefore, the detection signal V
s is an AC bias voltage that changes with the carrier wave. This AC bias voltage passes through the coupling capacitor C1 and the input resistor R11 and is input to the inverting input terminal of the operational amplifier OP1. Therefore, the operational amplifier output voltage Vo
Becomes an AC voltage that changes with the carrier wave centering on the reference voltage VREF3. When the infrared rays fluctuate, the pyroelectric infrared sensor S1 emits electric charges. This charge is the first FET1
Since the potential of the gate G of 1a is changed, the source current I
s1 changes. As a result, as the detection signal Vs, the center and the amplitude of the AC bias voltage change by a change corresponding to the change in the potential of the gate G. Therefore, as the operational amplifier output voltage Vo, the center and the amplitude of the AC voltage change by a change corresponding to the potential change of the gate G. Therefore, if attention is paid to the amplitude change of the operational amplifier output voltage Vo, infrared rays can be detected.

In the human body detection application, the frequency band of the signal is 1 Hz to 100 Hz, so the frequency of the carrier wave is at least 200 Hz or more, and practically 10
It is preferably about 00 Hz. So 1000Hz
Then, the low cutoff frequency fz of the amplifier circuit 20 is 10
It should be 0 Hz or less. The value of the coupling capacitor C1 is set to about 10 μF, and the input resistance R21 is set to 1.6.
When kΩ is set, the low cutoff frequency fz becomes 10 Hz by fz = 1 / (2π · C1 · R21), which sufficiently satisfies the above condition. On the other hand, if the feedback resistor R22 is 1.6 MΩ, the gain determined by the feedback resistor R22 / the input resistor R21 is 1000, which is a sufficiently high gain. Further, the startup time determined by the time constant τ = C1 (R21 + R22) is about 16 seconds, which is an allowable short time. Thus, in the infrared detection circuit 100,
It becomes possible to achieve both higher gain and shorter startup time.

[0014]

According to the pyroelectric infrared sensor circuit and the infrared detection circuit of the present invention, both high gain and short start-up time can be achieved. Therefore, it is particularly useful for applications such as human body detection for automatic door opening and crime prevention in buildings.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an infrared detection circuit of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional infrared detection circuit.

[Explanation of symbols]

 100 infrared detection circuit 10 pyroelectric infrared sensor circuit 11a first FET 11b second FET 12 constant current circuit 13 AC voltage generation circuit 20 amplification circuit S1 pyroelectric infrared sensor RG gate resistance RS1, RS2 source resistance Vs detection signal Vo operational amplifier output voltage C1 Coupling capacitor OP1 Operational amplifier R21 Input resistance R22 Feedback resistance VREF3 Reference voltage

Claims (2)

[Claims] 1. A drain of a first FET and a drain of a second FET are connected to a common constant current circuit to form a differential amplifier using each gate as an input terminal, and a signal from a pyroelectric infrared sensor is supplied. A pyroelectric infrared sensor circuit characterized in that a carrier signal is input to a gate of the first FET, a carrier signal is input to a gate of the second FET, and a detection signal is extracted from a source of the first FET. 2. The pyroelectric infrared sensor circuit according to claim 1, and an amplifier circuit for amplifying a detection signal from the pyroelectric infrared sensor circuit, wherein the amplifier circuit is a coupling capacitor. An infrared detection circuit, which is an inverting amplifier circuit including an input resistance, an operational amplifier, and a feedback resistance.