The circuit consists of a multivibrator (Ql & Q2), and a low power output stage Q3. The speaker should have an impedance in the region of 40 to 80 ohms. To use a low impedance speaker, connect an output transformer from the emitter of Q3 to ground. Cl and C2 determine the pitch of the siren and the values specified will provide a tone of about 300 Hz. Quiescent current is negligible.
The output at the collector of Q2 can also be fed into an amplifier input via a 1 Âµf electrolytic, in series with a 12 k resistor.