I had quite a few coin-cell lights, but theyād barely throw enough light to see the keyhole if kept on a keychain. I loved the idea, and had quite a few (shudder to think how manyā¦), but implementation left something to be desired.
Today, more efficient LEDs might make up for it, but back thenā¦ nah.
Also, getting squished would so easily turn them on and run them down.
So unless itās a deeply recessed button to reduce the chances of accidental activation (and even thatās no guarantee), I wouldnāt bother.
If I needed a smaller-the-better light, an AAA twisty would then be best for me.
Alas, Iām EDCing a UT20, a monster of an 18650 light. Doesnāt bug me. So AA, AAA, let alone coin-cell, isnāt that much of an issue.
I got two thingies that slip onto key-blanks (from the factory, non-serviceable) with a red and amber LED built-in. Uh, one each, that is. The red oneās dead at this point, and the amber barely glows. Then again, theyāre quite ancient. Those are nifty. Squeeze the key and the LEDās aimed right at the keyhole!
Had one, prehistoric era, with a bigger cell (PX-something? camera batteryā¦) with an actual hotwire bulb! Cellās replaceable, actually. Bāharni knows what I ever did with those (1 pristine, 1 quite āwornā). Separate units, would slide onto an existing key.
Those Iād see a market for, and would get an assload to gift to people, as theyāre quite useful.