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