DIY mains failure alert

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To give a audio or visual alert when the mains supply has failed and you need to take some action on the piece of aquarium equipment under monitor.

Why have one[edit]

Often a fuse in an item like a heater or pump will pop or the Mains will fail without you realising due to voltage surges during storms or high electricity use.


Mains absent alarm.png

Basically fitted between the Mains plug of the equipment to be monitored and where the lead goes into the device.


  • A Mains rated relay (SPST) capable of at least 0.2A on its contacts.
  • Terminal block to wire up the equipment lead to the relay coil.
  • 9V alkaline or lithium PP3 battery (for multi-year standby use).
  • PP3 battery clip.
  • Musical or alarm piezo buzzer capable of 6-12V operation. (Preferably consuming under 100mA so it lasts at least a couple of hours when on.)
  • 12V car type flashing LED with internal limiting resistor. (optional).
  • Plastic case to contain parts.


In normal use the relay is constantly energized by the mains and therefore the relay contacts keep the battery open from the buzzer or LED. When the Mains fails, the contacts drop and the battery can now power the buzzer and/or LED.