Security & Saftey

Security Alarm Circuit

Security Alarm Circuit

The Security Alarm Circuit is designed to safeguard your valuable documents and jewelry from potential intruders or theft. Simply place this circuit in front of your safe or beneath a doormat. When an unfamiliar person steps on the switch, the circuit will activate, triggering an alarm. What makes this circuit particularly advantageous is its ability to be employed in two locations simultaneously, with two distinct switches producing different alarm sounds.


  • Circuit Diagram of Security Alarm:
  • Circuit Components:
    • Working of Security Alarm Circuit:

Circuit Diagram of Security Alarm:

Security Alarm Circuit

Circuit Components:

  • Resistor
  • R1, R2 (100K) – 2
  • R3 (1.2K) – 1
  • R4 (47E) – 1
  • T1 (BC547) – 1
  • T2 (BC558) – 1
  • D1, D2 (1N4007) – 2
  • C1 (. 1uf) – 1
  • S1, S2 – 2
  • Speaker – 1


  • A resistor is a two-terminal passive device. They are mostly used in circuits to limit current flow across any of the circuits. The current flowing through the resistor is proportional to the voltage applied across the resistor’s terminals. Resistors are mostly divided into two types on the market:
  1. Fixed resistor–  It actually means that the resistor whose value cannot be change and remain what its mark on it.
  2. Variable Resistor-  It means that the value of resistance can vary within the range marked over it. For e.g. If the value of 5k is marked on it then it implies that the value of the resistor can vary from 0-5k.
  • The value of the resistor can be calculated either with the help of multimeter or with the help of color code over the resistor.

2. Diodes

A diode is a two-terminal device characterized by its asymmetric property, which permits the flow of current in one direction while impeding it significantly in the opposite direction. This ensures that current can exclusively move in a single direction, effectively blocking the alternate path. The diode features two terminals known as the anode and cathode. By leveraging the unidirectional behavior of diodes, it becomes possible to convert AC current into DC.

3. Transistor

A transistor is an electrical device with three terminals, employed to amplify weak input signals. It comprises two PN junction diodes connected in series. There are various types of transistors, including Bipolar Junction Transistors, Field-Effect Transistors, and Photo Transistors. Due to their compact size and low weight, they find widespread use in electrical appliances. Additionally, they exhibit energy efficiency, consuming less power for enhanced performance.

4. Speaker

It is a transducer which creates sound in reaction of the electrical auditory signal given in the input.

5. Capacitor

These two-terminal components, inherently passive, store electric charges. They utilize a dielectric medium to insulate and separate two conductors. The process begins when a potential difference between the conductors induces polarization in the dipole ions within the dielectric, thereby retaining the charge within the dielectric medium. In the market, you can find two main types of capacitors

  1. Polarized capacitor- Capacitor marked with – and + sign. They are mainly used to hold the charge. And before troubleshooting these capacitors carefully discharge them as they hold charges there is a risk of shock.
  2. Non polarized capacitor –  Capacitors which do not have any polarity marked over it. They are mainly used to remove the noises appeared while converting AC into DC.

Working of Security Alarm Circuit:

In the circuit, we have two switches, S1 and S2, which can be placed in two distinct locations – one in front of a locker and the other at the front door. When switch S1 is pressed, it initiates the conduction of diode D1, which is connected to it. Consequently, transistors T1 and T2, linked to a resistor, also begin to conduct. To enable oscillation, transistors T1 and T2 receive positive feedback from capacitor C1. The low-frequency tone emitted upon pressing switch S1 serves as an alert for the presence of an intruder.

Upon pressing switch S2, a similar scenario unfolds. Transistors T1 and T2, which are in the active state, receive power through diode D2 connected to switch S2. Consequently, sound is generated through the connected speaker. However, in this case, a high-pitched tone indicates the presence of an intruder near the locker. To silence the speaker, one must turn off the power supply, as it is the sole means to cease the sound emission.


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