Temperature-Compensated Capacitive Proximity Detector
This is a circuit of Temperature-Compensated Capacitive Proximity Detector. it will lights the LED when a person approaches the antenna plate. Here is the circuit :
The 4-inch-square antenna plate and a 33pF capacitor (C1) in parallel with 15pF (6 inches of coaxial cable at 30pF per foot) are used to form delay capacitance of this circuit. It charges to 5V via R5 during each positive half cycle of the input square wave. The capacitance equals 48pF and produces a delay of 16.5ns, when no body is near the detector. When there is a hand placed 6 inches from the detector, the capacitance rises to 50pF and produces a delay of 17.3ns.
Op amp IC2A is used to amplify and offset the DC voltage at TP1. As a comparator with hysteresis, this circuit uses the transistor and Op amp IC2B. The potentiometer (R2) is used to set a threshold other than 6 inches. To read out the proximity in inches this circuit uses a DVM at TP2. To ensure a well-defined transition, R2 is used. [Source: Maxim Application Note]