Make a power outage call annunciator using a buzzer with built-in music signal source

The circuit is shown in the attached figure. The circuit is powered by capacitor C1 for voltage reduction, 12V voltage regulator 1N4742 for voltage regulation, diode D1 for rectification, and capacitor C2 for filtering. When the call just comes in, since the voltage across capacitor C3 cannot change suddenly and is approximately 0, transistor V1 is cut off, and resistor R4 provides a sufficiently large base current to transistor V2, which is connected as an emitter follower and saturates V2, making the buzzer HA sound and informing the line of the incoming call. As C3 is continuously charged through R3, transistor V1 gradually enters the saturation state, and transistor V2 is cut off, and HA stops sounding.

When a power outage occurs, capacitor C3 discharges rapidly through diode D4 and resistor R2. Since the discharge time constant is very small, transistor V1 is cut off again in a very short time. At this time, capacitor C4 supplies power to transistor V2 and related circuits, causing V2 to be turned on again and saturated. HA sounds an alarm to inform that the line is out of power.

When the voltage across C4 is discharged to a smaller value, the buzzer HA stops sounding.

Adjusting the parameter value of resistor R3 or capacitor C3 can change the ringing time of HA when a call comes in; increasing or decreasing the capacity of capacitor C4 can change the ringing time of HA when a power outage occurs. Diode D2 can ensure that C4 only supplies power to the transistor V2 circuit when a power outage occurs, and diode D3 can appropriately extend the HA signal time when a call comes in.