@▷ Burglar Alarm with Expandable Modules | Diagram for Schematic

Burglar Alarm with Expandable Modules


The circuit has been designed to provide an alarm system against any form of burglary wherein it can be expanded by adding modules and zones as necessary.

TerminologyExpansion Module – a part or component used for convenience in adding or removing it in a deviceBurglar Alarm -  one way of preventing break-in’s into one’s property, not only to protect the possessions but the occupants as well; can prevent the havoc and disruption caused by theft and break-in’sZone – refers to an alarm control panel’s input from a protective circuit which divides the alarm system into separate independent areas of protection based on function which may include  medical, fire, intrusion, or critical condition monitoringCMOS 4001 – a quad 2-input NOR gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumptionBC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selectionsCircuit Explanation

The zone designed in this circuit contains automatic entry and exit intervals with a bell having its cut off timer. Utilizing a Personal Attacker / Tamper Zone, it functions on a 24 hour basis which makes the circuit conduct in always ON state. Triggering devices for normally open and normally closed circuits can be incorporated on this instant alarm zone. This protection is being supplied by a silicon controlled rectifier (SCR), a thyristor that conducts like a diode after a gate pulse is applied. It has ratings from 10 up to more than 5000 A, and from 200 V up to 6 kV. This SCR turns off when current becomes zero and prevents current flow until a pulse appears. Since it is widely used for controlled rectifiers, the SCR is found almost exclusively in power electronics applications, and is the most common member of the thyristor family. During the operation, once a switch opens from a normally closed loop, it will ring the main bell by triggering the SCR because of the current that flows through resistor R11. No time limit for the bell during this instance. Pressing SW2 will reset SCR by interrupting the current, so the normally closed loop can be restored.

The circuit is being powered by a 12 V supply. To start the circuit to function, the switch S1 should be set accordingly, but before doing this, all the LED should be properly lighting. By disabling the S1, the alarm will be armed. Once the switch has been set, there will be an allotted one minute to leave the premises which will trigger the buzzer to sound. It will only stop sounding when the door has been closed. In doing this procedure, it will ensure that the entry and exit operation has been restored effectively. Upon re-entering the premises, there will be one minute time allotment to turn OFF the switch SW1. Failing to do this will trigger and energize the relay and eventually ring the main bell, which will carry on for the span of 40 minutes. This can be deactivated at any time by turning OFF SW1.

There is an instant zone or access zone where there is no delay in the entry but will ring the bell upon closing the normally open switches. The same way applies to normally closed switches as they get opened. By placing a link between capacitor C7 and LED 3, it bypasses Q2, C8 and R8, and disables normally open switches.

Dividing the alarm into zones makes it easier to determine the origin of the fault or the cause of activation of zone. With the help of expansion modules, they can be added to the existing zone or simply replace them. The LED will turn red upon the activation of any zone until pressing the reset button. A single button can be shared among all the modules.


A burglar alarm system helps detect unauthorized entry onto a company's premises. The system sends a signal to a central monitoring center when activated. The monitoring centers provide 24/7 service and will alert the local police to dispatch authorities to the scene. It is a proven fact that the risk of burglary is significantly reduced after a burglar alarm is installed.

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