These days home security systems are given great importance everywhere. However these security automation systems are very expensive to implement. Here I will help you to make a simple IR security system by your own using an Infrared-photodiode pair.Earlier we had already published another intruder alarm circuit diagram which was based on LASER and LDR. However it has a probability that the thief might see the LASER beam and avoid theft alarm easily. This simple circuit will give you an insight into how do burglar alarms work.
Simple burglar alarm circuit diagram
- 555 timer IC
- IR led
- Transistor BC 548
- 12v Buzzer
- Resistor (100K, 1Kx 2)
- Capacitor (100uf).
Working of infrared alarm circuit
- Main component used here is a photodiode and IR led. We are creating a continuous loop arrangement around the house with it.
- 555 timer used here works in the monostable mode of operation. You can learn more about 555 monostable multivibrator working from our previous posts.
- IR led continuously strikes infrared beams on the photodiode. As a result the trigger pin (2nd pin) of 555 will get the positive voltage from Vcc. Hence it’s output will remain LOW as 555 monostable multivibrator gets a HIGH output only on negative trigger.
- When an intruder cuts the IR beam, photodiodes act as an open circuit. Thus 2nd pin gets negative through resistor R2 turning the monostabe output to HIGH.
- Output pin is connected to the base of a NPN switching transistor through a resistor R3. Thus the transistor becomes ON giving power to the buzzer.
- In case you need to ON some light bulbs or make a much powerful alarm bell, then you have to connect a relay instead of the buzzer.
- To increase the range of IR you can use a converging lens. How would you see if the IR beam has converged to the photodiode efficiently? For that you can use a camera or a visible LED to set the focus and then replace that with IR LED.
- Another method to increase the range is to use a series photodiode arrangement.
- You can use two 1.5 battery (AA size) to power the IR led.
- Photo diode should be connected in reverse bias mode (i.e. anode to negative and cathode to positive). Then it conducts when IR beams hits on it, else it acts as an ordinary diode.