Shadow Alarm

Listing ID #964142

Shadow Alarm detects moving shadow on LDR and starts a melodious sound alarm. This shadow alarm can be used as a simple burglar alarm that produces a loud beep when somebody crosses a protected area or door. The circuit is highly sensitive and can detect the shadow of the moving person from a distance of 1 meter. It does not require an aligned light beam to make the circuit standby. The beep generated from the circuit will be loud enough to detect the entry of a person in the room or the protected area being guarded. It is portable and can be places anywhere for monitoring.
About The Circuit :
The circuit uses a voltage comparator and a monostable timer to give the warning alarm on detecting a moving person. IC LM741 (IC1) is used as a voltage comparator with two potential dividers in its inverting and non-inverting inputs. Resistors R2 and R3 provide half-supply voltage of near about 4.5 volts to its inverting input (pin 2). LDR1 and preset VR1,R1 form another potential divider to provide a variable voltage input to the non-inverting input (pin 3). If VR1 is properly adjusted for the required light level, the output of IC1 will be high, which drives pnp transistor T1 out of conduction. This is due to the high potential at the base of T1. The emitter voltage of T1 will be high in this condition, which inhibits IC2 from oscillation and LED1 from lighting. IC2 is wired as a monostable timer. VR2 and C1 provide a preset time delay. As a person crosses the protected area, his shadow will be sensed by LDR1 due to change in the light intensity level and the voltage at the non-inverting input of IC1 will drop momentarily. The output of IC1 suddenly becomes low, allowing T1 to conduct. This triggers the monostable (IC2) and the alarm sounds. Adjust preset VR1 until LED1 turns off at the particular light level. Keep LDR1 facing the entrance of the room or the area to be protected. Sensitivity of the circuit depends on the proper adjustment of VR1. If VR1 is correctly adjusted, the circuit can detect a moving person from a distance of about 1-3 meters.