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L56, Power Supply Failure Alarm

This is the circuit design of power supply failure alarm. The vast majority of the power supply failure alarm / indicator circuits require an independent power supply for themselves. But the alarm circuit introduced right here requires no extra supply power source. It utilizes an electrolytic capacitor to hold sufficient charge, to supply power to the alarm circuit which sounds an alarm for a fair duration when the mains supply fails to deliver the electric power.


  • T1 = BC558B

  • D1-D4, D7, D8 = 1N4007

  • D5 = 15V 1W Zener diode

  • D6 = Red LED

  • R1-R2 = 82K 1W, R3 = 10K

  • R4 = 150ohm, R5 = 2K, R6 = 22K

  • R7 = 1.5K, R8 = 330ohm

  • C1 = 470uf, C2 = 100uf, C3 = 4700uf

  • BZ1 = Piezo Buzzer

During the existence of mains power supply, the rectified mains voltage is stepped down to a demanded low level. A Zener is applied to restrict the filtered voltage to 15-volt level.

Mains existence is recognized by a LED. The low level DC is utilized for charging capacitor C3 and reverse biasing switching transistor T1. Therefore, transistor T1 stays cut-off as long as the mains supply is existing. The moment the mains supply does not work out, the charge saved inside the capacitor works as being a power supply source for transistor T1. Considering that, while in the absence of mains supply, the base of transistor is pulled “low” through resistor R8, it conducts and sounds the buzzer (alarm) to provide an alert of the power failure.

Using the value of C3 as displayed, a high quality buzzer would sound approximately a minute. By increasing or decreasing the value of capacitor C3, this time could be modified for everyone demand.

The values of the parts will not be crucial. When the alarm circuit is operated from any external DC power supply source, the main supply section up to points “P” and “M” can be omitted from the circuit. Following points might be considered:

  1. 1. At a higher DC voltage level, transistor T1 (BC558) may possibly pass some collector-to-emitter loss current, producing a continuous murmuring sound from the buzzer. In that condition, substitute it with some low-gain transistor.

  2. 2. Piezo buzzer has to be a continuous tone version, with built-in oscillator.

To minimize the room space of this power supply failure alarm, one nicely make use of 5 small sized 1000uF capacitors (arranged in parallel connection) in place of capacitor C3 with bulky high-value.

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