1.) Seeing which color a random unlabeled clear LED is.
2.) Charging batteries, especially pesky 4.35V lithium ion cells that don't get full capacity from standard 4.2V chargers.
Seeing that I don't need fancy features like negative rails or lack of switching noise, I decided to build a cheap tiny "bench supply" using some Constant Current-Constant Voltage buck converter boards I found off Ebay and various voltmeters and ammeters, and then laser cut a case for it.
Here's your typical cheap buck converter board off Ebay.
The older ones have two trimpots (one to set voltage and one to set a current limit), while newer ones like above also have an additional pot to show the end-of-charge indicator LED (seeing as these boards are very frequently used as cheap CC-CV battery chargers). Some of the cheaper ones don't have current limiting, and only have a single trimpot for voltage. This one, being shipped from the US, is a bit more expensive at around $7 for a board, while those shipped from China can be had for $3-$4.
I had an older one that was previously used to drive LEDs.
The idea was to replace the trimpots with actual pots to control voltage and current, and add a voltmeter and ammeter, and then power the whole thing off some old 20V laptop power supply.
In one night I hashed together this unholy mess onto a random scrap piece of acrylic:
Being obviously extremely derpy, this clearly wasn't acceptable for long term use, so I decided to make something neater.
To power the voltmeter and ammeter, which run off of 5V, I found a 78L05 voltage regulator on the buck converter board (which probably powered the current-limiting circuitry) and a ran wire off it.
|Strain relief your wires kids|
For the pots, I bought your run-of-the-mill cheap 16mm linear pots off Tayda, in the same values as the trimpots. The banana plugs were crufted.
To make the case, I spent an afternoon making derpy CAD models of each component in Solidworks, and CADed a laser cut acrylic case. The case is held together with tabs and T-nuts.
I then used the Epilog laser cutter over at BU to make the case out of some 1/8" acrylic:
And then I assembled the case and inserted the components, bolting them on with M2 and M3 screws and nuts I had lying around. I unfortunately didn't have standoffs lying around, which I should have used.
To insert the tiny M2 nuts into the T-nut slots, I used a magnetic flathead screwdriver tip to hold the nut in place while I inserted the screw, like such:
A couple zip ties were used to clean up the wiring. Turns out my ammeter died and shows "10.7A" no matter how it's connected, which is unfortunate. Also, looking back, I definitely should have used multi-turn pots, as setting precise voltages is very difficult with pots which only have 270 degrees of travel.