Mains failure alarm circuit

admin March 8, 2012 26 Comments


Here is a simple mains power failure alarm/detector circuit that produces an alarm whenever the mains supply fails. Lot of such circuits are available, but the peculiarity of this circuit is that it requires no back up power source like a battery to power the alarm when the mains is absent.

When there is mains supply the transistor Q1(BC558) will be OFF and the capacitor C1 will be charged. When the mains supply fails the transistor Q1 becomes ON and the capacitor C1 discharges through the Q1 to drive the buzzer to produce an alarm. The capacitor C2 is the filter capacitor for the bridge. Diode D2 prevents the discharge of the C2 when mains fails. If D2 is not there,  the alarms will remain silent for a time capacitor C2 to fully discharge after the power failure.

 Circuit diagram with Parts list. 



  • T1  can be  a 230V primary   6V secondary  500mA  transformer.
  • B1 can be a 1A bridge.You can make the same using four 1N 4007 diodes.
  • All capacitors are rated 25V.
  • You can use any general purpose PNP transistor (like BC158,BC177  etc) as Q1.
  • October 8, 2012

    this circuit is wrong . it will always keep the buzzer on

  • September 14, 2012

    This circuit is wrong. I have edited this circuit and tested it is working fine. The buzzer will remain OFF if power is present. As soon as the power is down the buzzer will be switched ON till the capacitor is charged. No use of D2 and C2 here. Use a diode instead of series combination of (100 K pot and 10 K). connect the anode of diode to VCC and cathode to Base of PNP transistor. Use this circuit i have uploaded and edited

  • Pruthvi
    August 31, 2012

    plzzzz help me..
    I want to know the applications of ‘main supply fault detector alarm circuit.’
    Plzz respond for message soon

  • Pruthvi
    August 31, 2012

    i want to know the applications of ‘main supply fault detector alarm circuit’
    sir plzzzzz give me reply as soon as possible

  • maham
    July 2, 2012

    i have tried circuit but buzzer keeps on giving sound even when there is power supply

  • amrutha
    May 2, 2012

    this circuit gives an alarm even when power supply is on…capacitor of 470 micro farad is of waste..100k pot is not placed in correct position..don’t try this is just waste of time

    • kaptainpugwash
      February 11, 2013

      Got to agree with this. Replaced 470uF with 18000 uF gives about 2 mins of alarm. Unable to sort out the biasing so the alarm on all the time. Finally gave up with this, replaced it all with a small relay using the N/O contacts to sound the bleeper.Still fun waste of an hour.

      • kaptainpugwash
        February 11, 2013

        Sri should be N/C contacts as relay is energised when the power is on so bleeper sounds when power fails.

  • amrutha
    April 26, 2012

    reply soon plz

  • amrutha
    April 26, 2012

    what is the need of bridge rectifier

  • manju
    April 26, 2012

    sir,we are yet to start this project can you explain the working of the transformer,and why do we need 100K pot what is the necessary ?

  • amrutha
    April 26, 2012

    can you please explain the working of each part.and wat is the use of it

  • Kaliraj
    April 15, 2012

    Dear sir,i really appreciate your open sourcing mind..and i need a little help..i need a circuit which only produces a alarm for shorter delay say 20s when connected to mains..

  • Segzy
    November 29, 2010

    D rlay of hig&low cutoff only enrgis aftr 10s whn swtchd ON.whn vR1&VR2 is varied fails 2enrgis&dlay.hlp

  • June 20, 2010

    A simple diagram for 3 phase AC mains failure (audio with LED)alarm circuit.Inverter is also available i.e.Power for the said circuit can be from Inv. O/P.When the mains come and go, the alarm should work.Can you help?

  • saad
    April 12, 2010

    i have constructed the circuit but its not properly working the buzzer beeps when the power supply is provided but it doesnt beeps on failure please solve the problem

  • December 23, 2009

    can i use led istead of buzzer

  • Thomas
    June 27, 2009

    I have made this circuit but the buzzer keeps beeping while connected to the main supply. Can some help me on this??

  • Supun
    January 27, 2009

    What is The Purpose of D2 and C2.I feei nothing.

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