Condenser microphone (the alarm) mounted in the dog’s cage will perceive the sound bark and produces an AC signal, which pass through the DC blocking capacitor C1 to the base of transistor BC549 (T1). Transistor T1 with T2 will strengthen the voice signal and provide current pulses from the collector of T2. Trigger input pulses applied to the collector of transistor T3 and combined with capacitor C3 to the base of the transistor T4, which causes the T4 Cutt-off. The collector voltage of transistor T4 forward biases transistor T3 through resistor R8.
Barking Dog Alarm Circuit
Transistor T1 conducts and capacitor C3 discharges to keep transistor T4 cut-off. Transistor T4 remains cut-off until capacitor C3 charges enough to enable it to conduct. When transistor T4 conducts, its collector voltage goes low to drive transistor T3 into cut-off state. Resistor R9 and capacitor C3 are timing components. When fully charged, capacitor C3 takes about two minutes to discharge. So when sound is produced in front of the condenser mic, TRiAC1 (BT136) fires and the bulb (B1) glows for about two minutes.
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