This Circuit can compare the Light level in an area. It uses a PN Photodiode as the light sensor and IC CA3140 as voltage comparator. The circuit is ideal as the front end of burglar alarm circuits.
The circuit uses an Op Amp as voltage comparator. In voltage comparator mode, the OpAmp compares the voltage levels between its inverting input (pin2) and the non- inverting input(pin3) and gives an appropriate high / low output. Generally in voltage comparator mode, voltage at one input is kept fixed using a Zener diode or a potential divider resistor chain. In the circuit, the voltage at pin 3 is set by VR1 so as to keep the output high in a particular light level. The high output keeps the LED off. The PN Photodiode act as the light sensor. When the light reduces, current through the photodiode decreases. This increases the voltage at pin 2 of comparator and the output swings to low state. Current then flows through R2 and LED into the comparator and LED lights. This indicates, low light level or darkness.