This infrared detector circuit uses a passive IR-detector component PID20. It converts heat radiation into electrical impulses. The output voltage of the PID20 increases when an object comes close to it, this object must be warmer than the surroundings.
The voltage changes at the output of the IR sensor are sensed by the comparators IC2a and IC2b. Once the sensor detects an object (independent of the object’s actual temperature), one of the comparators will turn on the transistor T1. This transistor then closes the relay connected to it. A separate transistor for each comparator can be used so that the sensor will be able to determine whether the object is warmer or colder than the surroundings. The IR detector circuit consumes around 1 mA and the sensor consumes 0.2 mA.