Simple Darkness Activated Alarm

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Most darkness activated
alarms employ opamps and some logic ICs. Here, a less expensive approach
is shown based on the eternal 555, this time in monostable
multivibrator mode. Components R2 and C1 represent a one-second network.
When the LDR (light dependent resistor) is in
the dark, its resistance is high, pulling pin 2 of the 555 to ground.
This triggers the monostable and the (active!) 6-volt piezo buzzer will
sound. Preset P1 is adjusted depending on ambient light levels. The
circuit may be fitted on a wall in your home. Assuming P1 has been set
for the existing ambient light level, the shadow cast by anybody
entering the room or hallway will trigger the alarm.

Simple Darkness Activated Alarm Circuit

Simple Darkness Activated Alarm Circuit Diagram

Author: Myo Min – Copyright: Elektor July-August 2004