Modular Burglar Alarm Project

Description: This circuit includes automatic exit and entry delays along with a timed bell cut-off feature. It accommodates both normally-closed and normally-open contacts, and it has a 24-hour personal attack/tamper zone. The circuit is permanently connected to a 12-volt supply and is activated by opening switch SW1. Expansion modules can be utilized to add multiple zones, which may include inertia (shock) sensor types. All green LEDs should illuminate before SW1 is opened, allowing approximately one minute to exit the building. During this time, a buzzer will sound, ceasing once the door is closed, indicating that the exit/entry loop has been successfully restored. Upon re-entering, there is a one-minute window to turn SW1 off; failure to do so will energize the relay, activating the main bell for up to 40 minutes, although it can be silenced at any time using SW1. The "instant" zone operates without an entry delay. To avoid using normally-open switches, R8, C8, and Q2 can be omitted, and a link should be placed between LED 3 and C7. The 24-hour personal attack/tamper protection is achieved through an SCR/thyristor. If any switches in the normally-closed loop are opened, R11 will trigger the SCR, causing the bell to ring without a time limit. Once the loop is closed again, the SCR can be reset by pressing SW2, interrupting the current flow temporarily. The basic circuit is effective in many applications; however, fault detection is simplified when the alarm is divided into zones, allowing the control panel to track which zone triggered the alarm. The expansion modules are designed for this purpose, functioning alongside the existing instant zone but intended to replace it. When a zone is activated, its corresponding red LED will illuminate and remain lit until the reset button is pressed. All modules can utilize a single reset button.

The electronic schematic for this circuit features a robust design capable of handling various security scenarios. The core of the circuit is powered by a 12-volt DC supply, ensuring consistent operation across all components. The activation mechanism relies on the opening of switch SW1, which enables the system and initiates the exit delay sequence.

The circuit includes a microcontroller or logic circuit that manages the timing for exit and entry delays. This unit interfaces with the buzzer and the main bell, coordinating their operation based on the state of the exit/entry loop. The timed bell cut-off is implemented through a relay that engages when the entry delay period expires without the system being disarmed.

For the 24-hour personal attack/tamper protection, the SCR/thyristor is strategically placed within the normally-closed loop. This configuration allows for immediate detection of any unauthorized access attempts, with R11 providing the necessary current to trigger the SCR. The use of red and green LEDs serves as a visual indicator for system status, enhancing user feedback and operational clarity.

Expansion modules are designed to integrate seamlessly with the main circuit, providing flexibility to add multiple zones as needed. Each zone can be equipped with its own sensors, including shock or motion detectors, enhancing the overall security coverage. The design allows for easy troubleshooting and fault isolation, as each zone can independently signal an alarm condition while keeping the system operational.

In summary, this circuit presents a comprehensive security solution with features that accommodate various configurations and operational requirements, ensuring both user convenience and effective monitoring of premises.This circuit features automatic Exit and Entry delays and a timed Bell Cut-off. It has provision for both normally-closed and normally-open contacts, and a 24-hour Personal Attack/Tamper zone. It is connected permanently to the 12-volt supply and its operation is "enabled" by opening SW1. By using the expansion modules, you can add as many zones a s you require; some or all of which may be the inertia (shock) sensor type. All the green LEDs should be lighting before you open SW1. You then have up to about a minute to leave the building. As you do so, the Buzzer will sound. It should stop sounding when you shut the door behind you. This indicates that the Exit/Entry loop has been successfully restored within the time allowed. When you re-enter the building you have up to about a minute to move SW1 to the off position. If SW1 is not switched off in time, the relay will energise and sound the main bell. It will ring for up to about 40 minutes. But it can be turned off at any time by SW1. The "Instant" zone has no Entry Delay. If you don`t want to use N/O switches, leave out R8, C8 and Q2; and fit a link between Led 3 and C7. The 24 Hour PA/Tamper protection is provided by the SCR/Thyristor. If any of the switches in the N/C loop is opened, R11 will trigger the SCR and the bell will ring. In this case the bell has no time limit. Once the loop is closed again, the SCR may be reset by pressing SW2 and temporarily interrupting the current flow. The basic circuit will be satisfactory in many situations. However, it`s much easier to find a fault when the alarm is divided into zones and the control panel can remember which zone has caused the activation.

The expansion modules are designed to do this. Although they will work with the existing instant zone, they are intended to replace it. When a zone is activated, its red LED will light and remain lit until the reset button is pressed. All the modules can share a single reset button.