A low-cost electronic alarm circuit diagram

This is a low cost, simple yet powerful electronic siren that is powered by only a 9V battery. This circuit can provide the final circuit block module in an alarm circuit using a relay to activate it.

How a Simple Electronic Siren Works

When the switch is pressed, C3 charges through R4 with a time constant of 0.47 seconds. When the switch is released, C3 begins to discharge more slowly through R7 and R3 with a time constant of about 5 seconds. The op amp is set up as a voltage controlled oscillator. The control voltage in this simple electronic siren circuit is the exponential rise and fall of the voltage as C3 charges and discharges.

When the output of the oscillator (pin 7) goes low, the charge remaining on C1 keeps pin 5 below the switch point. The current through R7 is proportional to the control voltage on C3. This current discharges C1, causing the voltage on pin 5 to rise toward the switch point at a rate proportional to the voltage on C3. When the switch point is reached, pin 7 switches high and initially pulls pin 6 high through C1. This causes the op amp to temporarily hard start. But C3 quickly charges through D2, causing the voltage on pin 5 to drop below the switch point and causing the op amp to shut down again.

The positive pulse output of the op amp places a fixed amount of charge into C2, slightly raising the potential at pin 6. This causes the potential at pin 6 to rise and helps the op amp turn off quickly. Also, R5 and C2 delay the rise at pin 6 long enough to get a good output pulse.

The cycle then repeats. However, during the C3 discharge cycle, the charge rate of C1 decreases with each repetition of the oscillator (because the control voltage is lower), and the output frequency decreases accordingly. During the C3 charge cycle, the opposite happens.

The output pulse is buffered by a second op amp which then applies current to the drive transistor. The output waveform has a low duty cycle but produces a very loud tone.

This simple LM358 based electronic siren kit is now available. Download the PDF version with the parts list