Expandable Alarm Circuit: Build Your Zone-Based Security System

Description

The Basic Alarm Circuit incorporates an Automatic Exit/Entry Zone – an Immediate Alarm Zone capable of accepting both normally-closed and normally-open triggering devices – and a “Constant Vigilance” 24-hour Personal Attack/Tamper Zone. Utilizing Expansion Modules – it’s possible to add as many supplemental alarm zones as needed.

More about components

Integrated circuits, or ICs, are crucial components within this circuit. Specifically, the system utilizes an SCR/Thyristor for the 24-Hour Personal Attack and Tamper protection. These semiconductor devices are designed to control high-power electrical circuits. An SCR/Thyristor acts as a solid-state switch, controlling high currents. It allows current to flow through a circuit when a specific voltage threshold is reached and then automatically turns off when the voltage drops below a certain level. This precise control is essential for managing the alarm’s response to intrusion attempts. Furthermore, ICs such as transistors are used for switching and amplification within various sections of the circuit, ensuring reliable operation and responsiveness to the triggering devices.

The alarm is armed and disarmed via SW1. Prior to moving the switch to the “set” position – all green LEDs should illuminate. Subsequently, you have approximately one minute to vacate the building. During this period, the Buzzer will activate. The sound ceases once the door is closed behind you, signifying successful restoration of the Exit/Entry loop within the allotted time.

Upon re-entering the building – you have approximately one minute to shift SW1 to the “off” position. If SW1 isn’t switched off within this timeframe – the relay will energize, and the main bell will sound. The bell will continue to ring for up to 40 minutes. However, it can be silenced at any time by SW1.

The “Immediate” zone lacks an Entry Delay. As soon as one of its normally-open switches is closed – the main bell will ring. Similarly, when one of its normally-closed switches is opened – the main bell will ring. To avoid using normally-open switches – eliminate R8, C8, and Q2, and create a link between LED 3 and C7.

The 24-Hour Personal Attack and Tamper protection relies on the SCR/Thyristor. If one of the switches within the normally-closed loop is opened – the current through R11 will trigger the SCR, and the main bell will ring. In this scenario, the bell operates without a time limit. To reset the PA/Tamper zone – initially restore the normally-closed loop – then briefly press SW2. This interruption of current resets the SCR.

Parts list

Two-Zone Expansion Module

The basic circuit proves satisfactory in numerous situations. However, when protecting a large building – it’s easier to pinpoint the source of a fault when the system is divided into zones, and the control panel can remember which zone triggered the activation.

These expansion modules are designed to achieve this functionality. Although they operate with the existing immediate zone – they are intended to replace it. When a zone is activated – its red LED illuminates and remains lit, indicating the zone’s activation status.

The intention is that – once you’ve identified the zone – you then press the reset button and deactivate the LED. The reset button simply turns off the LED; it doesn’t reset the zone. The zone resets automatically when the trigger circuit is restored. If multiple expansion modules are utilized – they can all share a single reset button.

Inertia-Sensor Module