Light/buzzer error indicator using battery

Thread Starter


Joined Nov 13, 2012
I have an electrical box powered by 120Vac, and want to have a red LED indicator light turn on if ever the 120V power is gone. A battery would have to be on stand-by to turn on this light.

My plan is to have the 120V terminals connect to a 24V battery charger charging a 24V battery, and a relay that closes contacts from the battery to the light, and probably a little piezo buzzer, powered by 24V, when no voltage is detected from the source. So the light & buzzer is like an error indicator. I called a battery supplier, they said it's better to use lead acid battery (b/c it's hard to get lith-ion phosphate batteries shipped) and a UPS board so that back-current won't occur just in case the 120V power goes out.

>What should I look for when charging this battery? What should I be careful of (too little voltage, too much, etc.)?
>Should I use a UPS board? What exactly do they do?
>What type of battery should I use that could last a few years?


Joined Sep 9, 2010
First of all, there are likely some decent commercial products you could buy off the shelf for this. I'd start there.

You can get sealed lead acid batteries or nicad batteries which would be fine for this job. You don't need much capacity if all you want is a buzzer and an LED, unless you want the alarm to run for days. So, I'd think about 2-4 AAs. You could use a small charger meant for 1) the battery chemistry and voltage you choose, and for 2) continuous duty.

Detecting a loss of power should be easy. I think you could just look for a loss of the DC voltage coming from the charger. This would trip a comparator, which in turn would trip a MOSFET and turn on your LED and buzzer circuit.

But again, don't build something you can just buy. (Unless you want to!)