This is the 555 based light sensitive alarm circuit. It can be also called as solar car alarm circuit. When night changes to day, the alarm sounds. This can be used in closed garages. If anybody opens the garage, the light falls in the alarm and the alarm sounds.
There are two photo diodes D1 and D2. The two diodes are connected to two 555 IC’s. IC1 functions as a Monostable Multivibrator and IC2 as Astable multivibrator. When light falls on D1 or D2, it triggers the monostable multivibrator IC, ie., IC1, and IC1 gives a trigger pulse to IC2. When IC2 receives the trigger signal from IC1, IC2 generates square pulses as IC2 functions as a Astable multivibrator. The frequency of the pulse production is given by the formula below the circuit. According to the pulses, the audio transformer T1, T2 generates audio, and so the speaker ‘B’ which is connected to the end of the circuit sounds.