The working of this circuit is that, when any one crosses before the pyroelectric infrared sensor, then the alarm or chime sounds. So, this circuit can also be called as a dual purpose circuit, which can be used as a alarm circuit or doorbell circuit. This circuit finds its application mainly for security purposes.
In the circuit diagram, BISS0001 is an PIR sensor IC. It is a connected with motion sensor(PIR 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 IC4. When the signal comes, either chime circuit or alarm circuit closes depending upon the connection, which is already set manually. The alarm and chime controls are denoted in the circuit itself.
So, this circuit can be used for automatic counter also.