Burglar alarm circuit or Theft alarm circuits are available in several forms. The above circuit was one of that kind using timer IC 555 as the main component. The circuit is very easy to construct and the cost of this circuit is also very less. So we can implement this circuit by our own and install this circuit in our home so that you can improve your knowledge and it also adds protection to your home.
WORKING OF BURGLAR ALARM CIRCUIT:
The working of this circuit is very easy to understand.In the above circuit IC 555 was used as a Astable multivibrator.The Astable multivibrator puts out a continuous stream of rectangular pulses having a specified frequency. Thus by using the Astable multivibrator we can produce continuous frequency which was given as input to the speaker which makes an alarm sound in case of any disturbance in its trip wire. Pin diagram of IC 555 was given below for your better understanding.
Now we can see how we can change the Astable Multivibrator to a alarm circuit.In the above circuit the 4th pin (RESET pin) was connected to the ground using the thin strand of copper wire which is used as a trip in this circuit.The function of the 4th pin that it resets the timing interval when it was connected to the ground.
When the wire was connected the Multivibrator cant produce the rectangular pulses of constant frequency.When the copper strand is cut off then the 4th pin will have no connection with the ground.Therefore IC 555 produces the output pulse and which was given as input to the speaker thus it drives the speaker to give a noise which would scare the burglar away.Thus your homemade effective burglar circuit is ready