Power failure alarm circuit sharing

This is a "special" circuit design for power failure alarm. Most power failure alarm/indication circuits require their own independent power supply. But the alarm circuit presented here does not require additional power. It uses an electrolytic capacitor to maintain sufficient charge to power the alarm circuit, which will sound for a considerable period of time when the mains power cannot provide power.

During the presence of mains power, the rectified mains voltage is stepped down to the required low level. A Zener diode is used to limit the filtered voltage to the 15 volt level. The presence of power supply is identified by an LED. Low level DC is used to charge capacitor C3 and reverse bias switching transistor T1. Therefore, transistor T1 remains off as long as the main power supply is present. At the moment when the mains power is not working, the charge stored in the capacitor acts as the power supply for transistor T1. Consider that, in the absence of power supply, the base of the transistor is pulled "low" through resistor R8 and it will conduct and sound a buzzer (alarm) to provide a power failure alert.

Using the displayed C3 value, a high-quality buzzer will sound for approximately one minute. This time can be modified according to each individual's needs by increasing or decreasing the value of capacitor C3.

Building the circuit is very simple and easy. The value of the parts doesn't matter. When the alarm circuit is powered from any external DC source, the main power supply section up to points "P" and "M" can be omitted from the circuit. The following points can be considered:

At higher DC voltage levels, transistor T1 (BC558) may pass some collector to emitter loss current, producing a continuous buzz from the buzzer. In this case, replace it with some low gain transistors.

The piezoelectric buzzer must be a continuous tone version with a built-in oscillator.

To minimize the room space of this power failure alarm, a good idea is to use 5 small 1000uF capacitors (arranged in parallel) instead of the bulky high value capacitor C3.