The working of this circuit is that, when any one crosses before the pyroelectric infrared sensor, then a signal can be send through a radio transmitter and it can be received at a very long distance in a receiver. So, this circuit can be used for security even in remote areas.
In the circuit diagram, B2288 is an PIR sensor. It is a motion sensor, which detects the movevment of human. This sensor can be placed in an security needed place. When anyone moves, immediatley the PIR sensor gives a signal to IC1, and IC1 passes the signal to IC2.IC2 ie, TWH8778 is a switching IC. WHen the signal comes through control pin 5, it immediately closes and the signal can be send to Encoder IC, VD5026. When this encoded signal reaches the relay TD400, the relay switches ON, and it passes the encoded signal to the transmitting antenna.
In the receiver side, there will be decoder which decodes the signal from PIR sensor. This circuit is mainly used in ARMY, NAVY and in country borders for security purposes.