Expandable SCR Burglar Alarm Circuit Diagram & Build Guide

Description

This is a straightforward SCR-based security system circuit. It incorporates automatic Exit and Entry delays alongside a timed Bell deactivation and Reset function. It’s designed for integration with typical normally-closed input devices, such as magnetic-reed contacts, micro switches, foil tape, and PIR sensors.

The fundamental alarm configuration features a single area with "Exit/Entry" delays – which may suffice for numerous applications. However, larger structures are preferably divided into multiple zones. The adaptable design allows for the addition of any desired number of zones to the system. These “Instant” zones can be activated by both normally-open and normally-closed input devices.

More about components

The core of this circuit relies on an SCR (Silicon Controlled Rectifier), a semiconductor device utilized to control high-power circuits. The SCR acts as an electronic switch, controlling the flow of electricity based on a gate signal. When the gate signal is applied, the SCR turns ‘on’, allowing current to pass through. When the gate signal is removed, the SCR returns to its off state, stopping current flow. SCRs are frequently used in power control applications, such as switching relays, motor control, and lighting systems. They are valued for their fast switching speeds and ability to handle high currents. The specific SCR used in this circuit allows for the control of the alarm’s output, facilitating the activation of the buzzer and siren.

Furthermore - the circuit utilizes a relay – a magnetically operated switch. The relay serves as a switching mechanism to control the buzzer and siren. The SCR activates the relay, and the relay, in turn, closes the circuit to the output devices, triggering the audible alarms. This configuration offers a reliable and adaptable method for implementing the alarm’s functionality. The selection of the relay contributes to the overall performance and responsiveness of the security system.

It’s user-friendly. Upon powering the alarm, you have approximately 30 seconds to vacate the building. When you return and open the door – the Buzzer will activate. You then have about 30 seconds to deactivate the alarm. If this isn’t achieved – the Siren will commence sounding.

After approximately 10 minutes - the alarm will attempt an automated reset. If the trigger circuit has been restored – the attempt will be successful. However – if the loop remains open – the attempt will fail - and the alarm will re-activate. Naturally – you can silence the Siren at any point by disabling the alarm.

A standard bell typically consumes around 400mA. An electronic siren generally draws less current. If you plan to draw a heavier current from either the Buzzer or Siren terminals – the SCR in question should be fastened to a metal heatsink – and the relay contacts may require upgrading.

If you prefer to omit the timed “cut-off and reset” feature – leave out D5, D6, R11, R12, Q3, Q4, C6 and the Relay.

Due to manufacturing variations - the precise duration of any delay depends on the characteristics of the actual components you’ve utilized in your circuit. Nevertheless - by modifying the values of R2, R7 & R11 – you can adjust the Exit, Entry and Reset times to align with your particular requirements. Increasing the values extends the time – and vice-versa.

Stripboard Layout

Parts List