A highly sensitive human proximity detector

This detector has a simple circuit, high sensitivity and temperature compensation function. The circuit is shown in the figure.

The integrated operational amplifier, gate circuit, and resistor-capacitor components in the figure constitute the human proximity detector circuit, in which the inverter T1, 1MHz crystal oscillator, and resistor-capacitor components constitute a 1MHz oscillator. The oscillation signal is shaped by T2 to output a square wave signal. A 25cm2 copper-clad plate is used to form the sensor. When the human body approaches the copper-clad plate, the human body is equivalent to another plate, and a capacitance of 2 to 6pF will be generated as the human body approaches. This capacitance increment will cause the subsequent IC3 to output a pulse signal with a pulse duty ratio proportional to the capacitance increment. The square wave output by T2 is input to the comparator ICIB through R3. After amplification, it is added to one input end of the XOR gate IC3; the other path is first delayed by R1 and C1, and then amplified by the comparator ICIA and added to the other input end of the XOR gate IC3, so that the duty ratio of the square wave signal output 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 distance of the human body.

In actual use, a slight change in the transmission delay of the IC1 comparator itself may mask a slight change in the capacitor C1. Therefore, this circuit uses a dual comparator circuit with the same characteristic parameters. It can distinguish time differences below nanoseconds and offset the effects of its own transmission delay, offset voltage, temperature drift, etc., so that the output signal voltage at the TP point is only proportional to the delay of R1 and C1.

The capacitor Cl in the figure actually includes three parts: a 33pF fixed capacitor, a 25cm2 sensing plate-to-ground capacitor and a coaxial cable input capacitor. When no one approaches the detector, the sum of the three capacitors in parallel is about 48pF, which will produce a fixed delay of 0.69R1C1=16.5ns; when someone approaches the sensing plate, the capacitance increases to 50~54pF, which will produce a delay of 17.3~18.6ns. The time difference between the two square waves is 0.8-2.1ns, which can guarantee the required conversion sensitivity for a dual comparator that can distinguish time differences below nanoseconds.

The function of the op amp IC2A is to provide bias and amplify the DC voltage at the TP point. By adjusting the potentiometer R11, the threshold value corresponding to different distances of the human body approaching the sensing plate can be set. The op amp IC2B and the power amplifier tube 01 form a hysteresis comparator to amplify the output voltage of IC2A. The positive feedback of R14 can ensure that Q1 is turned on quickly and reliably to light up the LED. If the LED is replaced with a relay, when the human body approaches, the relay will operate and control various other actuators to achieve various automatic control functions.