Electronic siren circuit

  
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This is a compact electronic siren circuit based on three transistors. This circuit is suitable for in corporating with other alarm or siren projects such as burglar alarms, automatic factory sirens etc or a simple push to on alarm. The electronic siren circuit given here is based on a complementary transistor pair consisting of Q2 & Q3 (BC557 & BC
Electronic siren circuit - schematic

37) wired as an astable multivibrator oscillator, which directly drives the speaker. The transistor Q1 is used toprovide a full chargeon capacitor C2 when power is turned ON. When push button switch S1 is pressed, the capacitor C2slowly discharges through resistor R8. This makes the circuitto oscillate at a low frequency that increases to a high frequency and kept indefinitely as the capacitor is fully discharged. When the switch P1 is released, the output frequency decreases slowly as C2 is charged to the positive voltage through resistance R6 and the Base-Emitter junction of tramsistor Q2. When C2 is fully charged to the positivebattery voltagethe circuit stops oscillating.



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