The automatic modulated siren circuit shown in the figure really produces an interesting sound effect, which can be adjusted at three different points. In this circuit, the first unijunction transistor (Q1) produces the modulation signal set at P1. This signal is of the "sawtooth" type and its frequency also depends on capacitor C1. The second unijunction transistor (Q2) produces the audio tone, but its frequency is also controlled by the first oscillator and is basically adjusted in P2. The depth of the effect is adjusted in P3. Combining the three settings we can obtain very interesting sounds for this circuit. The modulated audio signal is applied to the power transistor that excites the speaker. This circuit can be used as an excellent performance automotive siren. The 10 ohms resistor at the source of the power transistor can be changed depending on the desired sound power.
