Pyroelectric human body induction switch

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Pyroelectric human body induction switch [Copy link]

The pyroelectric human body induction switch circuit
uses a pyroelectric infrared probe and amplifies the weak signal received by the probe, and then drives the relay to make a pyroelectric human body induction switch. It can be used in energy-saving automatic switches of electric lights, automatic doors, safety protection, anti-theft and other equipment.
[Circuit working principle]
This circuit uses LN074B as a probe. When the probe receives the pyroelectric infrared signal released by the human body, it is converted into a weak low-frequency signal with a frequency of about 0.3~3Hz by the control head. After being amplified by the two-stage amplifier VT1 and IC2, it is input to the voltage comparator IC3. The two-stage voltage amplification uses a DC amplifier with a total gain of about 70~75 decibels.
IC3 and others form a voltage comparator, where RP is a reference voltage adjustment potentiometer, which is used to adjust the circuit sensitivity, that is, the detection range. Normally, the reference voltage (IC3 (2) pin voltage) is higher than the input voltage of IC2 (IC3 (3) pin voltage), and IC3 outputs a low level. When someone enters the detection range, the probe outputs a detection voltage, which is amplified by VT1 and IC2 to make the signal output voltage higher than the reference voltage. At this time, the (6) pin of IC3 outputs a high level, the transistor VT2 is turned on, and the relay J1 can be electrically attracted to turn on the switch.
In the circuit, VT3, C7, R8, and ~R10 form a power-on delay circuit. When the power is turned on, the induction of the person turning on the power will cause IC3 to output a high level, causing false triggering. At the moment of powering on, the power-on delay circuit turns on VT3 due to the charging effect of capacitor C7, so that the high level output by IC3 is connected to the ground through VT3, and VAT2 can remain in the cut-off state, preventing false triggering of the power on. The power-on delay time is determined by the time constant of C7 and R8, which is about 20 seconds.
[Component selection] The pyroelectric infrared probe uses LN074B type. IC2 and IC3 use high input impedance operational amplifier CA3140. This circuit uses a junction field effect transistor as a differential input stage, with an input impedance of up to 1.5*10(12) ohms, an input offset current of only 0.5pA, a bandwidth of 4.5MHz, and a conversion rate of 9V/us. It is an operational amplifier with excellent performance and is very suitable for the amplification stage of weak signals.
The probe is installed at a height of about 2 meters from the ground. When designing the shell, the lens should be at a depression angle of about 13 degrees to the ground, so that a monitoring area can be formed. Since the control angle of the detector is only about 86 degrees, the best angle should be selected during installation to minimize the dead zone.
[Circuit debugging]
Circuit debugging mainly involves adjusting the potentiometer RB and selecting a suitable reference voltage to achieve the best sensitivity.

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