Author Options:

Extemely simple automatic emergency light. Answered

I'd like some help to settle an argument regarding my extemely simple automatic emergency light.

According to my friend, who installs emergency lighting, this circuit is too simple and has too many components missing but I say he's wrong, if one part fails it can easily be replaced. I'm not and electronics expert either.

I'll explain the circuit...

5v comes from a 5v 2A phone charger.
Battery is a 32Ah 12v SLA. 12v charger is a smart charger with built-in status monitor.
SPDT 5v relay rated to switch 12v @ 17amp max
The light is a 12v 5 metre LED Strip with built in current-limiters and draws arount 2A.

Basically, 5v powers the relay which means the battery is connected to the smart charger that keeps it topped up while there is mains power (240v here in UK). If there is a power cut the relay turns off and battery is switches over to the LED light Strip. Its that simple.

It works as designed, no parts have failed and I haven't found any signs of over-heating. My friend believes it should all be one circuit and not seperate parts and is a fire risk as it is.He just won't let it go, so does he have a point or is he just being a/an _________ (fill in the blank, lol)? I've included a schematic of the circuit. Not sure if I've drawn the relay properly though.



5 months ago

The most commercial ones work straight from 230V. They have a Schmitt-trigger (in some way) that senses the voltage, they include some electronics for "self-test", they monitor the batteries on reguar bases. The most simple one, like yours: one contact closes when the power fails, closing a circuit that drives LEDs or another light source. According to some laws (in Belgium) it should last for at least one hour. So your circuit rocks!


Reply 4 months ago

I've tried leaving the emergency light lit for 3 days straight before it began to slightly dim and only 2 out of 192 LED (roughly 3.5 metres strip) have failed. Was going to check the amperage until i found my meters screen is cracked.

It may have run for longer but 3 days seems long enough for now.

Personally I'd say its a success and possibly out does Belgium's laws.


5 months ago

For domestic purposes what you have built seems fine, The relay's and chargers are sufficient enough to do the job. I think any problems you have will be maintenance, such as relays which will fail from mechanical wear over time and obviously the battery will degrade. Commerical units have a level of inbuilt protection by self-testing and make use of opto-isolated SSR, voltage sensing and battery regulation to provide a guaranteed level of operation and life span, which you need if lives depend on it's operation.


6 months ago

I dismantled quite a few emergency lights over the years.
Mostly to replace old batteries but often to upgrade them from the tiny halogen models to LED ones.
You and your friend are both right...
From a hobby level point of view your circuit does exactly what it is intendet to do.
From a commercial of actual safety point of view however it would be totally useless.
Let me exlpain some details so you settle your arguement for good:
The main difference to a commercial circuit:
There is no battery protection, meaning no really regulated way to charge the battery and check the charge level.
The relay should be a transistor of sorts.
Mains power should be checked and used to charge the battery but all needs to be seperated by opto couplers or similar.
What you did is to simply eliminate all vital components that make a commercial model pass the tests and requirements.
The circuit uses a 5V wall wart, so no need to use opto couplers as there is no main voltage going anywhere.
The relay does what it should although not as reliable (in the long term with many outtages) as a transistor - but with a socket a replacement is a matter of seconds.

As Rick pointed out: What matters is that it works for you and your purpose!
A simple improvement might be use a SSR if there is a need for higher amps but otherwise I would say: If it does for you what you need it to do then you must be right ;)


Reply 5 months ago

Regarding the amps, battery was left over from a previous scrapped project and is the only one I have. After you mentioned using an SSR I'm now considering it. Unfortunately I didnt have any at the time I made the circuit.


5 months ago

tl;dr: You've transferred the risk of battery maintenance to the maker of the charger, so it's as safe/reliable as it's UL/CE/whatever rating. The dead man's switch you've designed is as reliable as the relay you've used. K.I.S.S. at it's finest. Nothing wrong with that.

Big Clive has several Youtube videos on emergency lights that are very informative. In particular
which explains the regulations that cause it to be more complex.
This one which is his own design and a step between your "dead man's switch" and a commercial device:

And even an teardown of an American version of which I have 2 or 3 of the same model where I work: All worth a watch. He gives a great detailed teardown as well enough practical explanation for an electronics neophyte to follow along.


6 months ago

1. Does it work
2 No - there is something missing
3 Yes - fine carry on.

The definition of QUALITY in a product is that it does what the end user wants.

There may be many more components in a commercial product for various reasons, legal or practical - However something that works to your satisfaction is a success on any level.

There are many ways to skin a cat.