Help: building circuit to switch on emergency 12V light

I need some build help from an electronic fundi for a circuit.

I’d like a LED 3W 12V downlight to come on if the main power fails - to run off an existing 12V 7ah battery that’s already being kept topped up by solar.
I’ve seen some circuits on the interwebs but I’d prefer this from someone who already knows their stuff than me hacking away at it.

Can anyone help with the build, I don’t mind paying?


Maybe @tangmonster911 can help you

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Simplest method I can think of would be to have a relay (of course rectifier, etc.) which would remain open while mains is on. Once mains fail, rectifier closes & light is powered…

Finer details would of course matter (current via mains, etc), but simplicity almost always produce the best result


electro-mechanical is the simplest option.


it all depends on the solar panel you are using (particularly how much overhead is available to run the standby circuit)

for a lower standby current, a comparator holding a mosfet gate to ground, would also work.

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Good inputs - thanks everyone appreciated
The circuit @angryplum shows is what my interwebs research led to too … however the 12V battery is already being charged via the solar system (3 x 300W panels for the Geyserwise geyser system) so charging via the mains seems redundant?

Or am I reading the diagram above wrong and there’s no charging happening via the above? Wonder why the LED is noted as ‘charging indicator’ ?

this diode carries the charging current through the relay into the battery. just DC where X marks the spot.

now the left half is just a boolean reference (is there mains power) and a power source to keep the relay open.

IMHO using a resistor to power a 3W led is, albeit cheap and functional, a cop out.

get a 3W led driver circuit on bidorbuy for R25, it will not only protect the led from transients but also the led wont dim when the battery voltage drops.

Report back: built a circuit after purchasing the components from Yebo Electronics this morning and all working fine.
Great to have the stairwell lights now on backup to prevent falls if the power goes out.

Thanks to all for the help and insights :thumbsup:

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Cool! Dont forget to tell us what you ended up doing.

Would be interested in seeing the final diagram

Here you go:

The cellphone charger didn’t really need the bridge rectifier but I put it in as I was unsure of it’s quality. Otherwise it’s just a simple relay circuit using the 6V side as a mains sensor.
The 12V side gets charged by the Geyserwise system, and if the power fails then this relay will parasite off the backup battery. I calculate it will keep the single 3W LED downlighter over the stairs going for 10+ hours so its more than enough to make sure no-one falls in the dark down the stairs till we get the paraffin / gas lamps up

One correction: there’s no 1K resistor on the 3W LED circuit as those downlighter bulbs run directly with their own circuitry from a AC/DC 12V source. I was just lazy to edit that part of the pic


if the downlight was designed to run off a 12v transformer it probably has a limiting resistor of some sort integrated, negating the need for the 1k 3w resistor in series. if the cellphone charger has a dc output then the FWBR is not required and would only serve to drop the input voltage to the relay coil by 0.8 - 1.4v depending on the diodes. but if the relay energizes correctly, then it should be no problem.

glad you got it figured out

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Spot on - I did see the voltage drop after the bridge rectifier but the relay coil managed to still engage well with a solid “click” so it was ok in the end

This all has got me interested in what else in the home automation I might do…

One thing I would like to try is reading and logging temperatures/water flow/rainfall etc now. I’m leaning towards a Raspberry Pi and some SQL with ESP8266 and sensors

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