Light-sensitive Alarm Project
A kit for this project is available from
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The circuit detects a
shadow falling on the light-sensor and sounds the bleeper when this
happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The
bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be
used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking
this beam will then cause the bleeper to sound. The light sensor is an LDR (light-dependant
resistor), this has a low resistance in bright light and a high resistance in dim light.
The light-sensitivity of the circuit can be adjusted by varying the 100k preset.
The length of bleep can be varied from 0.5 to 10 seconds using the 1M preset.
Using the 7555 low-power timer ensures that the circuit draws very little current (about 0.5mA)