Build Your 20-Minute Battery Burglar Alarm Schematic

Description

This single-zone security system incorporates independently adjustable Exit, Entry, and Siren Cut-Off timers. It’s designed to accommodate standard normally-closed input devices, such as magnetic-reed contacts, foil tape, and PIR sensors.

Upon activation of the alarm, the Siren will emit sound for a duration of up to 20 minutes. Following this, it will automatically cease operation and remain inactive. The system will not reactivate itself.

Should you prefer, a mains power supply can be utilized. However, the exceptionally low standby current makes battery operation a viable alternative. A 9-volt supply was employed in the drawing; nevertheless, the circuit functions effectively with voltages ranging from 5 to 15 volts. Integrated circuits play a crucial role in this device. Specifically, a 555 timer IC is utilized to generate the delay signals required for the exit and entry timers. This IC is a versatile timing circuit, capable of creating precise delays by controlling the flow of current through a transistor. The 555 timer’s internal circuitry generates a square wave signal whose frequency is determined by the values of external resistors and capacitors, allowing for accurate timing of the delay periods. Furthermore, a CD4017 decade counter IC is incorporated to manage the siren cut-off timer. This integrated circuit functions as a digital counter, producing a unique output pulse each second. By connecting it to a relay, it enables precise control of the siren’s duration. These integrated circuits contribute significantly to the system's functionality and allow for tailored control of the various timer functions.

The Support Material for this alarm includes a comprehensive circuit description, a parts inventory, a step-by-step construction manual, and additional resources.

Schematic Diagram

Operating this alarm is straightforward. Sw1 may be any kind of switch, either single-pole or multi-pole. If the Buzzer emits sound when activated by Sw1 - the normally-closed loop is open. Switching Sw1 off again and inspecting the building for open doors or windows allows for verification of the loop's integrity. If the Buzzer does not sound, the loop remains intact.

Depending on the setting of R3, a timeframe of approximately one minute is provided for exiting the building. During this period, the Buzzer will signal the event. Upon closing the door behind you, the Buzzer should cease sounding, confirming the loop has been restored within the allotted time.

Upon your return and opening the door, the Buzzer will reactivate. The duration of this activation is determined by the setting of R4, providing up to approximately one minute for deactivation. Should this time elapse without intervention, the Siren will initiate operation.

The duration of the Siren’s operation is governed by the setting of R5, allowing for a period of approximately 20 minutes. Following this, the Siren will automatically shut off and remain inactive. Naturally, the noise can be stopped at any time by repositioning Sw1 to the "off" position.

For this type of apparatus, extremely precise timings are not necessarily required. Should you prefer, the potentiometers may be replaced with fixed resistors. For example, 2M2 resistors can deliver exit and entry delays of roughly 30 seconds, and a Siren cut-off time of approximately 10 minutes.

Following the cut-off timer’s deactivation of the Siren, the Buzzer will continue to sound. Therefore, upon your return, if the Buzzer is emitting sound, you’ll know that the alarm has been engaged.

The Support Material for this alarm includes a detailed circuit description - a parts list - a step-by-step guide to construction - and more.

Stripboard Layout