Light-controlled Alarm

Project Number: 106

OBJECTIVE: To show how light is used to turn on an alarm.

Description: The alarm will sound, as long as light is present. Slowly cover the photoresistor (RP), and the volume goes down. If you turn off the lights, the alarm will stop. The amount of light changes the resistance of the photoresistor (less light means more resistance). The photoresistor and the transistor (Q2) act like a dimmer switch, adjusting the voltage to the alarm. This type of circuit is used in alarm systems to detect light. If an intruder turned on a light or hit the sensor with a flashlight beam, the alarm would trigger and probably force the intruder to leave.