Using the PID20 circuit, manufactured by Siemens (which converts thermal radiation into electrical impulses), an operational amplifier and a few electronic components can be made very simple infrared detector.
Size of the output signal of PID20 is determined by the task at pins 3 and 4.
The output signal at pin 3 is compared with a reference voltage equal to half the supply voltage. Reference voltage is taken from the voltage divider R2-R3-R4-R5. When approaching an object warmer than the surrounding environment, or to remove an object colder than the environment, the output voltage increases.
Variation of the sensor output will be compared, the IC2a and IC2b, located voltage of 0.5 V under and over voltage reference respectively. Depending on the output, one of the comparators basculate and it opens on T1.