The small electronic circuit presented here is a low cost electronic siren. It produces a periodical sound of varying frequency, like a siren but do not expect high fidelity. The generated audio signal is an almost unfiltered square wave, rich in harmonics.
The electronic siren produces about 1W of audio power which is enough for small, battery operated alarm systems. The circuit is based on the popular 555 timer. It consists of two multivibrators units, both based on 555. The first multivibrator oscillates at a low frequency and produces a square waveform. The square waveform is then converted to a triangle wave from R3 and C2 and this waveform modulates the second oscillator which runs at a higher frequency. There is also a small power amplifier based on a Darlington pair used to amplify the generated sound wave and drives a small speaker. Volume can be adjusted from RV1 potentiometer. D3 is used for DC restoration and biases the Darlington pair.
If you decide to drive a power amplifier with this siren, it is better to use a LPF filter in order to improve sound fidelity by suppressing harmonics. In this case, it is better to drive the LPF filter from C3 and omit the darlington pair and D3 diode. You can also adjust the sound of the siren according to your preferences by replacing R1, R2, C1 or R4, R6 and C9. R1, R2, and C1 determine the speed of frequency sweep and R4, R6, C9 determine the central frequency of the generated sound.
