This Circuit can monitor the door steps or Staircase. When a person crosses the steps, the alarm sounds indicating the entry. The circuit is too sensitive and operates in day light.
The circuit uses an NPN Darlington phototransistor L14F1. It senses the intensity of light through its exposed base and passes current in the collector- emitter path. When there is light, the phototransistor conducts and C1 charges up to its full voltage level. The alarm circuit uses a Monostable timer built around IC NE555. The triggering threshold of IC1 is adjusted using VR1. So normally the trigger pin 2 of IC1 remains high as set by VR1. When the shadow of the moving person masks the phototransistor, it turns off allowing C1 to discharge through R1. This momentarily makes the trigger pin 2 of IC1 low and the timing cycle starts. With the given values of R2 and C2, Buzzer sounds for two minutes.
The circuit can be constructed on a small piece of Perf board. The circuit is designed to operate in day light. Switch off the circuit in the evening to avoid false alarm. If a lamp is provided opposite to the phototransistor on the opposite wall of staircase, the circuit can be used as day and night alarm. Align the lamp in the line – of – sight of Phototransistor. Adjust VR1 till the circuit becomes silent.