Simple Burglar Alarm Circuit using Transistors

Source:EFY    

Description:

The burglar alarm described here is a simple circuit that automatically sounds an alarm whenever an intrusion is detected.The system is implemented by installing a sensing loop, which is basically a thin copper wire. This way, the loop is both invisible and brittle enough to break easily and initiate the alarm mechanism.

At the time of an intrusion, the loop breaks and this causes the loudspeaker to generate a continous and loud alarm sound. Once the loop is broken, the alarm can only be silenced by either turning OFF the power supply or by replacing the broken wire by a new continuous loop.

Part List:

Semiconductors:

T1 - Transistor BC548

T2 - Transistor SK100

Resistors(all 1/4W, ±5% carbon):

R1 - 270 Kilo- ohm

R2 - 10 Kilo-ohm

Capacitors:

C1 - 0.1μF,Ceramic disk

Miscellaneous:

S1 - SPDT Toggle(On/Off) Switch

LS - 8Ohm,1Watt Loudspeaker

Battery Holder

Printed Circuit Board(PCB)

Working Principle:

The circuit employs a simple mechanism to sound an alarm when an intrusion is detected. It consists of an oscillator section which is merely a two-stage amplifier with feedback. This amplifier is implemented with the help of two transistors(T1 is a NPN transistor-BC548 while T2 is PNP transistor-SK100) connected in a feedback arrangement to generate oscillations. Transistor T1 is responsible for generating oscillations while transistor T2(connected to the base base of T1) amplifies these oscillations in order to drive the loudspeaker for sounding the alarm.