Need to design a simple and useful security alarm then you can refer this IR Based security Alarm using 555 timer prototype. In this circuit IR (Infra Red) beam acts as a Invisible fence, whenever the IR Rays are blocked then this circuit will produce alert sound through loud speaker.
This circuit has two stage, first one is IR emitter & IR receiver stage then second one is Astable Multivibrator timer circuit, Here the timer circuit Reset pin is Accessed by the IR stage hence the multivibrator oscillates when the IR rays blocked.
- IR LED Photo diode pair
- IC 555
- Loud speaker 8Ω
- Resistors 220Ω, 10KΩ, 1KΩ, 150KΩ each one
- Variable Resistor 10KΩ
- LED green
- Capacitors 0.1μF = 2, 0.01μF
- Transistor BC547
- Battery 9V
Construction & Working
Here IC 555 timer is configured in Astable multivibrator mode and Reset pin is connected with Q1 transistor collector terminal, this Q1 transistor acts as a switch and connects the Reset pin to Ground if the IR rays received by photo diode and connects with positive supply if there IR rays blocked.
The IR LED gives IR rays continuously and it should be placed towards photo diode in straight line.
Here the timer IC output frequency is depends on the timer Resistor R4, VR1 and timer capacitor C2, by varying VR1 we can get different level output sound at loud speaker.