Burglar alarm circuit or Theft alarm circuits are quite common and different circuits uses different methods to detect intruders. The above circuit uses timer IC 555 to detect and sound alarm upon detecting intruders. The circuit is very easy to construct and the cost less. In order to detect intruders this circuit uses a trip wire which should be tied in the path where we need to detect intruders. This may be entrance of our house or the hallway door etc. When intruders move in the trip wire breaks or disconnect from the circuit. This will trigger the alarm to sound alerting the user and scares away the intruders.
WORKING OF BURGLAR ALARM CIRCUIT:
In the above circuit IC 555 acts as a Astable multivibrator, you can read the working of IC 555 here. The thing about Astable multivibrator is that it puts out a continuous stream of pulses of specific frequency. Thus by using the Astable multivibrator we can produce continuous frequency which will be used as input to drive the speaker.
If you know about Astable multivibrators. you will be aware of the fact R2, R3 and C2 decides the output frequency. This is given by the formula
F = 1.44 / (( R3 + 2R2)x C2 )
= 218 Hz
Now there is one variation in the above circuit from usual Astable multivibrator setup. The 4th pin of IC is an active low pin which means when low signal is applied to it the output of 555 will be inactive or low. When reset pin is connected to logic 1 or high signal output will become active. And in the above circuit you can notice the 4th pin (RESET pin) was connected to ground using the thin strand of copper wire which will be our trip wire. Also the reset pin uses a pull up resistor to connect to Vcc.
Here when the trip wire is intact and not broken. The reset pin will be in low state and therefore output will be inactive. When intruder entering our house breaks this trip wire, R1 pulls the reset pin to high logic and immediately the output will become active. As a result 555 timer will produce pulse signal in its output. This will drive the speaker connected to it, a capacitor of 100uF is used to remove DC elements in the signal.
The speaker we use here is 8 ohm 1 Watt. 555 timer IC output current at 12V will be around 100mA and the output voltage will always 1.3v less than Vcc. Therefore output voltage will be 10.7v. Calculating the power output from 555 IC
P = V x I = 10.7 x 100mA = 1.07 Watt.
Therefore 555 timer IC can drive the output speaker safely and gives a loud audible alarm sound fitting to use in this alarm.
Up to $20 shipping discount on first order now: https://jlcpcb.com/quote