This 555 timer Photo alarm circuit is based on a 555 timer IC and a LDR (light dependent resistor ) .The resistance of the LDR ( photoresistive cell) varies inversely as the light intensity , the resistance is high when the intensity of light is low ( illumination level is low ) .
The LDR enables the alarm when light falls below a certain level. The certain level can be set by modify by turning the variable resistor R1 . The LDR used in this project is a cadmium sulphide (CDS) photoresistive cell . The photoresitive cell used in this circuit has a resistance about 3 megaohms in darkness and 100 ohms in bright light .For this LDR electronic circuit you can use a wide range power supply from 5 to 15 volts .