Build Your 12V Burglar Alarm Circuit Diagram

Description

This is a single-zone security system – featuring automatic exit, entry, and siren deactivation timers. It’s compatible with standard input devices, including magnetic reed contacts, foil tape, and passive infrared (PIR) sensors. However, it’s easily adaptable for using normally-open triggers.

When the alarm is triggered, the siren activates for a predetermined duration. Subsequently, it ceases operation and remains inactive. The alarm will not reactivate automatically.

I’ve utilized a 12-volt power supply in this diagram; however, the circuit can function with voltages ranging from 5 to 15 volts. Simply select a suitable siren, buzzer, and relay for the chosen voltage.

Schematic Diagram

When the alarm is switched on, the buzzer emits eight distinct beeps. This represents the exit delay. Before the completion of the eighth beep, one can safely exit the building without triggering the alarm.

R6 controls the length and tempo of the beeps. It can be adjusted to provide an exit delay ranging from approximately ten seconds to around a minute. Following the eighth beep, the buzzer should cease sounding, confirming that the loop has been restored within the allotted timeframe.

If the buzzer continues to emit a constant beep instead of stopping, the loop is open, and the building is not secure. When this occurs, switch off the alarm and check for unlocked doors or windows.

Upon your return and opening the door, the buzzer will sound again, initiating the entry delay. The entry delay mirrors the length of the exit delay. To distinguish it from the exit delay, the buzzer emits a continuous sound.

If the buzzer is sounding continuously, the alarm has been activated, and the entry delay has commenced. If you don’t deactivate the alarm before the entry delay expires, the siren will activate.

The siren triggers only once. Similar to R6, it governs the duration of both the exit and entry delays, as well as the siren cut-off time. The siren cut-off delay is thirty times the length of the exit delay. With an exit delay of thirty seconds, the siren will sound for approximately fifteen minutes before it shuts off and remains inactive.

Of course, the noise can be stopped at any time by switching off the alarm.

Following the cut-off timer's deactivation of the siren, the buzzer will continue to sound. Therefore, when you return, if the buzzer is still active, you’ll know that the alarm was previously triggered.

Note that D10 is optional. Its function is to consistently sound the buzzer during the entry delay. It also ensures that the buzzer remains operational after the siren has been deactivated.

If D10 is omitted, the buzzer will emit eight beeps during both the exit and entry delays. And, when the siren cuts off, the buzzer will also terminate.

Alternative Capacitor

A standard electrolytic capacitor is polarised. If the charge on its plates is applied in the reverse direction, DC current will flow through the capacitor. If this current is sufficiently high, the capacitor will heat up and potentially explode. The presence of R5 within the circuit prevents this from occurring. However, if a polarised capacitor is utilized, the oscillator may not operate reliably, or it may not operate at all.

While the oscillator is in operation, the polarity of the charge on C4 periodically reverses. Therefore, C4 must be non-polarised. However, a non-polarised 10µF capacitor can be simulated by connecting two 22µF polarised capacitors in reverse polarity, as shown.

Stripboard Layout

Parts list