It’s really easy and quite enjoyable to add LED features to Xmas and birthday cards, and they will literally run for weeks on a single set of batteries. Note that I’ve nothing against the red cross and it’s valuable volunteer workers. Just the issue that a significant percentage of their income goes to the management who then use those volunteer workers to do the actual work.
The LED tester is a very common unit on eBay. They’re quite handy to have. Here’s a general purpose eBay search link:-
All the LEDs used in this project should be matched if using them in parallel with a single resistor. For other colours you could use individual resistors, although the LED with the lowest forward voltage will tend to slowly drag the battery pack voltage down below the point where the others can light.
When testing the LEDs for balance the one that is brightest of the parallel clump is the one with the lowest voltage, so it steals more of the current.
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