Have idea, need guidance

Having recently upgraded the doorbell transformer to support my smart doorbell, I had to pull the original chime out of the wall and now have a large hole in the drywall in an excellent position for an emergency lighting fixture. While I was expecting that this was a need that someone would’ve already come up with a product to fill, I am unable to locate an emergency lighting fixture that will run off the 16V/30VA power that’s available in this hole.

Question 1: is this a stupid idea? i.e., would it make sense to build an emergency lighting fixture that charges a battery and turns on when, based on the reading from a light sensor, the power supply drops?
Question 2: if it’s not a stupid idea, any guidance on similar projects that would help me get started on using the right materials and components for a device that I won’t have to worry is going to catch fire?


Don’t think it’s a stupid idea at all. In fact, I have the same situation and may just do the same.
I assume the input voltage of the transformer is your home power 120/220. I would check what circuit it is on and how it is protected.

Then I would remove the transformer and wire an emergency light directly to the input AC. Here is an example:


It would probably cost you $30-50 for the parts including batteries, charger, and power outage monitoring. This might be the simplest and cheapest.

Unless you just wanna build one and in that case, I would copy one of the commercial designs.

If you want to build one then Google “DIY Emergency Light” there are lots of examples. If you want to have some fun with embedded control it would be built from something like; an Arduino + current sensor+ charge controller+ LION battery+ power converter + Mosfet LED driver + LED strings.

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the hole in question is about 20’ and down a floor from the transformer (2 stories + attic, transformer is in the attic). Wiring appears to be cat 3 phone type cable, so putting 120 on it is pretty out of the question. It might be possible to follow the cat3 thru the ceiling into the hole with a run capable of holding 120, though, so not a crazy suggestion.

wonder if the cat3 jacket is strong enough to use to pull through a piece of romex :slight_smile:

Unless it was old work, even low voltage doorbell wire is normally stapled. I wouldn’t try to pull it through unless it were loose… If it’s loose, I’d use it to pull through pull tape, and use the pull tape to pull the romex.

I made a little bridge rectifier for my 24VAC furnace transformer then fed it into a buck converter to power an Arduino so I could add a timer just for a timed furnace fan/recirculate option.

You might be able to do similar and trickle charge an emergency light if you find the right one with appropriate charge characteristics as can be pulled from your 16/30VAC xformer.

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