Security alarm using IC 555

We know that when pin-4 is connected to positive potential, the IC works in normal condition, but whenever, the pin-4 is connected to ground, it overrides all functions of the IC and the output of the IC remains LOW. This facility is used in the following circuit to design a simple security system.

Circuit diagram

When the RESET pin-4 is returned to ground, through a loop wire, the IC is disabled and whenever the loop is broken, the alarm sounds up.


When supply is connected to the circuit, keeping the loop-wire intact, pin-4 of the IC remains grounded. So the IC is disabled and its output remains LOW. So the speaker remains silent and this condition remains as it is till the loop-wire is broken. The moment the loop-wire is broken, pin-4 of the IC is returned to positive potential and the IC comes in action.

Now capacitor C1 starts charging through series resistors R1 and R2 and discharging through R2 only. Due to this, a high frequency signal i.e. square wave (about 1.5kHz) is produced at output pin-3 of the IC. The output contains DC component also. So a decoupling capacitor C3 filters out the unwanted DC component and only the square wave signal is connected to the speaker. So the speaker produces a shrill whistle like alarming sound.

For more information about internal structure of IC 555, also refer this article.

Important points to note

  • The loop-wire can be fitted across a door or any other entrance, which you want to make secure. The loop-wire may be very thin.
  • You can use up to 40 SWG wire, with a maximum length of 2 meters as a loop-wire.
  • DO NOT INCREASE THE LENGTH FURTHER. This is because if the resistance of the loop-wire is sufficiently large, it will form a potential divider with resistor R3 (100k – connected between pin-4 and positive line) and the IC may accidentally enable producing alarming sound.
  • For more information and understand the working of potential divider circuit, refer my book SEMICONDUCTOR DEVICES & CIRCUITS.
  • You can change the frequency of the circuit by considering different values of R1 and then calculate the values possible related values of R2 to suit your required frequency value. For calculating different values of R1, R2 and C1, visit this IC 555 calculator.

Possible modifications

Instead of using loop-wire between pin-4 and ground, you can use microswitch across the the two points. Then connect the micro switch behind a door in such a way, that when the door opens the micro switch becomes OFF. This will trigger the alarm.


  • Best suitable for any type of security system in which you want to monitor the invading of unauthorised persons.

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