The circuit detects a sudden 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.
- resistors: 10k, 47k, 1M ×3
- presets: 100k, 1M
- capacitors: 0.01µF, 0.1µF, 10µF 25V radial
- transistor: BC108 (or equivalent)
- 7555 low-power timer IC
- 8-pin DIL socket for IC
- LDR (light-dependant resistor) type ORP12
- bleeper 9-12V
- on/off switch
- battery clip for 9V PP3
- stripboard 12 rows × 25 holes