Simple and practical human proximity detector circuit diagram

1206RGB (single)

Test probe P100-M3

This paper introduces a human proximity detector with simple circuit, high sensitivity and temperature compensation function. The circuit is shown in the figure.

The integrated operational amplifier, gate circuit and RC component constitute the human body proximity detector circuit. The inverter T1, 1MHz crystal oscillator and RC components form a 1MHz oscillator, and the oscillating signal is shaped by T2 to output a square wave signal. A 25cm2 copper plate is used to form the inductor. When the human body is close to the copper plate, the human body is equivalent to the other plate, and the capacitance increases with the proximity of the human body, and the capacity is 2-6 pF. This increase in capacitance causes the subsequent IC3 output pulse duty cycle to be proportional to the capacitance increment. The square wave of the T2 output is input to the comparator IC1B via R3, amplified and added to one input terminal of the XOR gate IC3; the other path is delayed by R1 and C1, and then amplified by the comparator IC1A and added to the XOR gate IC3. The other input terminal is such that the duty ratio of the square wave signal outputted by the XOR gate IC3 is proportional to the delay of R1C1 in the input circuit, and then filtered by R6 and C4 to generate a DC voltage proportional to the proximity distance of the human body.

In actual use, the small variation of the IC1 comparator's own transmission delay may mask the small change of the capacitor C1. Therefore, this circuit uses a dual comparator circuit with the same characteristic parameters, which can distinguish the time difference below nanoseconds, and The effects of its own transmission delay, offset voltage, temperature drift, etc. are offset, so that the output signal voltage of the TP point is only proportional to the delay of R1 and C1.

In the figure, the capacitor C1 actually includes a 33pF fixed capacitor, a 25cm2 sensor board ground capacitance and a coaxial cable input capacitor. When the unmanned proximity detector is used, the sum of the above three capacitors in parallel is about 48pF, which will generate 0.69R1C1. Fixed delay of=16.5ns; when someone approaches the sensor board, the capacitance increases to 50~54pF, which will produce a delay of 17.3~18.6ns. The time difference of two square waves is 0.8~2. 1 ns, for dual comparators that can resolve time differences below nanoseconds, is able to guarantee the required conversion sensitivity.

The function of the op amp IC2A is to provide a bias and amplify the DC voltage of the TP point. The adjustment potentiometer R11 can set a threshold value corresponding to different distances of the human body approaching the sensor board. The operational amplifier IC2B and the power amplifier tube Q1 form a hysteresis comparator, which amplifies the output voltage of the IC2A. The positive feedback of the R14 ensures that the Q1 is quickly and reliably turned on and the LED is lit. If the LED is replaced by a relay, when the human body approaches, the relay acts, and various other actuators can be controlled to realize various automatic control functions.