Emergency LED Lights - Power Loss Detection

So recently there was a tornado watch storm that hit 90mph in my area late night/early morning. My ADHD son was going nuts as he is afraid of the dark.

I did some research into emergency lighting and found most required to be hard-wired into my home's wiring (at a cost of $200+ an hour for electricians to do, its not in my budget).

So I remember going through some basic electronics book when I was younger and remember some basic circuitry that stays 'open' when there is a current provided (low) and is 'closed' when the current cuts out.

I found a cheap $5 outlet tester in home depot which has a few LEDs on it to let you know if the outlet is active and correct and I believe I can use the use the 'active' state of those LEDs to provide this signal to keep a circuit open until power loss occurs.

So my question is, (as my memory fails me) how would I construct this, as I only remember it having something to do with a very basic transistor or MOSFET (or both?)... The LED light would be battery powered (like a work light)

Your advice and thoughts?

You could use a relay with the battery operated lamp on the NC connection.

Then energise the relay coil with an AC/DC wall wart thingy of a voltage that suits the relay coil. When the mains is on the coil is energised and contacts open, so the lamp on the NC side goes off. Mains goes off, coil no longer energised, contacts close, lamp comes on.

Thanks manor_royal - are all relays normally closed and open when energized? Is there a specific name for this type of relay other than normally closed specification?

They usually (afaik) have a NO and NC connection so you can wire them as on (and energised off), or off (and energised off).

Thanks manor_royal,

Wouldn't a transistor work in the same way? I use the low power of the LED line to switch on the power of the battery line?

oh wait... I think you mean I won't need that power line tester then... with the relay and a wall wart...

You could sense the mains using a chip like HCPL3700, circuit described here https://forum.arduino.cc/index.php?topic=435506.0 part is available at digikey.com

When AC loss is detected, have your Arduino turn on a transistor like AOI510, also at digikey, to sink current thru the battery powered LED lights.

(50 to go ...)

Thanks CrossRoads,

While I understand this is an Arduino forum - I do believe this basic circuit wouldn't necessarily require the Arduino, unless I had other features in mind.

probably just an AC Adapter->Relay(NC)->Switch on Light that has a battery source

I suppose.


Yup - Thanks for the battery charging thought - now I need to find some inexpensive, intelligent charger to integrate :-p

You should also incorporate some kind of daylight sensor. Otherwise the lamp will come on if the power goes off in the day and drain the battery.

You guys are awesome! - thanks!

oh wait… I think you mean I won’t need that power line tester then… with the relay and a wall wart…

Exactly. You use the NC contact of the relay to switch the light’s power, and wire the relay coil to a cheap DC wall wart like a phone charger or something. When you have power, the relay coil is energized and the switch is held open. When you lose power, the wall wart stops working too and the coil is deenergized, causing the switch to spring closed and allow power to the light.

The technical term for what manor_royal is referring to is called double throw. A switches electrical arrangement is classified by the number of poles and throws it has.

Throws refers to the number of positions a switch can have. There will be one common pin, which is connected to one of the other contacts depending on the switch position. Single throw switches have only one path that is opened or closed with the switch actuation. Double throws (also called changeover switches) have two outputs, and switches with 3 or more positions are often called selectors. The speed selection switches on cheap floor fans are examples of 3 or 4 throw switch.

Poles refers to the number of electrically separate switch components are actuated together by the same mechanism.

For 1 or 2 poles and throws, S (single) and D (double) are used to describe the number. T (triple) might be used for 3, but at 3 and above it’s very common to just use the numeral.

Some switches can have ridiculous numbers, like this 6PDT switch.

Technically you only need a SPST NC relay for this, but don’t waste your time looking for a single throw relay, double throws are much more common.

One other point, make sure it’s a non-latching relay.

Very informative, Jiggy-Ninja! Thank you!