Security & Saftey

Security Alarm Circuit

This Security Alarm Circuit will assist you in protecting your valuable documents and jewellery from intruders or theft. All you have to do is position this circuit in front of the locker or beneath the mat, so that if an unfamiliar person walks over the switch, the circuit will activate and an alarm will ring. The circuit’s key advantage is that it may be inferred in two places at once, as two distinct switches make two different noises.


  • 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
  1. Resistor: 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 device with two terminals that has an asymmetric property, meaning that it allows current to flow in one direction while high resistance flows in the other. As a result, current can only flow in one direction, and the other path is blocked. Anode and cathode are the two terminals of a diode. With the use of diode unidirectional behaviour, AC current can be converted to DC.

3. A transistor is a three-terminal electrical device that is used to boost weak input signals. A transistor is made up of two PN junction diodes coupled in series. Bipolar junction transistors, Field effect transistors, and photo transistors are examples of different types of transistors. Because of their small size and light weight, they are frequently used in electrical appliances. Furthermore, they use less energy and hence are more efficient.

4. Speaker- it is a transducer which creates sound in reaction of the electrical auditory signal given in the input.

5. Capacitor– Electric charges are being stored by these two terminal components which is passive by nature. A dielectric medium is used which is used to separate two conductors. It started at the time when the potential variation occurs in the conductors polarizes the dipole ions to hold the charge in the medium which is dielectric. There are two varieties of capacitor available in the market –

  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:

S1 and S2 are the two switches in the circuit that can be utilised in two distinct locations, one in front of the locker and the other on the front door. When the switch S1 is pressed, the diode D1 connected to it begins to conduct, and the transistors T1 and T2, which are connected to the resistor, begin to conduct. Transistor T1 and T2 receive positive feedback from capacitor C1 for the purpose of oscillation. The low tone frequency generated when switch S1 is pressed indicates the presence of any intruder.

When switch S2 is pressed, the same situation occurs. The transistors T1 and T2, which are in the waking state, receive power from the diode D2, which is connected to the switch S2. As a result, sound is produced by the speaker attached to it. However, in this case, a high-pitched tone is heard, indicating that an intruder is present near the locker. The only way to silence the sound coming from the speaker is to turn off the electricity.


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