The purpose was to obtain more power than the siren circuit already available since many years,One-IC two-tones Siren and to avoid the use of ICs.
A complementary transistor pair (Q2 & Q3) is wired as a high efficiency oscillator, directly driving the loudspeaker.
Q1 ensures a full charge of C2 when power is applied to the circuit. Pressing on P1, C2 gradually discharges through R8: the circuit starts oscillating at a low frequency that increases slowly until a high steady tone is reached and kept indefinitely. When P1 is released, the output tone frequency decreases slowly as C2 is charged to the battery positive voltage through R6 and the Base-Emitter junction of Q2. When C2 is fully charged the circuit stops oscillating, reaching a stand-by status.
A good sized loudspeaker will ensure a better and powerful output tone.
As stand-by current drawing is zero, SW1 can be omitted and B1 wired directly to the circuit.
Maximum current drawing at full output is about 200mA.
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