Power supply failure alarm

The Power Supply Failure Alarm indicates power supply failure by sounding a buzzer. The circuit requires no external DC supply and the power-on condition is indicated by a red LED. No. of ICs used: None The Power Supply Failure Alarm employs an electrolytic capacitor to store adequate charge to sound an alarm for a reasonable duration when the mains supply fails. During the presence of mains power supply, the rectified mains voltage is stepped down to the required voltage level. A zener diode is used to limit the filtered voltage to 15V level. The presence of AC mains is indicated by a red LED. The low-level DC is used for charging a capacitor (C3) and reverse biasing the switching PNP transistor BC558 (T1). Thus, transistor T1 remains cut-off as long as the mains supply is present. As soon as the mains power fails, the charge stored in the capacitor acts as a power-supply source for transistor T1. Since, in the absence of mains supply, the base of transistor is pulled ‘low’ via a resistor (R8), it conducts and sounds the buzzer to generate an alarm indicating of power-failure. Learning: Application of diodes in the bridge rectifier arrangement. Use of zener diodes for voltage regulation. Application of PNP transistors for switching purposes Brand: Kits'n'spares, Marketed by: Kits'n'spares