This circuit sent out an alarm when its LDR sensor is exposed to light by sun or lamp. A 555 astable multivibrator was used here which sent signal a tone of about 1kHz upon detecting light. The sensor when exposed by light completes the circuit and makes the 555 oscillate at about 1kHz with transistor to drive current. Sensitivity can be adjust wit
h P1. This makes the sun light to flow through it to the ground and prevents the alarm from going on due to the stored light on the sensor.