Sorry that this took a year to figure out, but I've finally come up with a design that might not be totally useless, but easy to build and rather inexpensive: An LED tester and polarity finder.
When I had to handle 0201 LEDs, I quickly learned that they don't really come with useful polarity markings. Yes, there's something on the bottom, but when I try to flip them around again even with my best tweezers, chances are that they end up in any orientation. So I need to find polarity while the LED is already facing up.
It's also a bit hard to decide on a current limiting resistor before knowing how an LED will look when a given current is flowing.
So here's a polarity tester that allows to find the right orientation AND the right current limiting resistor:
- multiple pads to place the LED on, with the large ground pad to the left. The right row of pads is supplied through a set of resistors
- resistor values are chosen such that the current even from 2 or 3 pads won't destroy an LED rated for 20 mA
- pads are close enough to test packages as small as 0201, but large enough to test up to 1206
- supply through pin header (3...5 V), USB (5 V obviously), or a 2032 cell on the bottom through a diode to avoid reverse current when another source (pins or USB) is plugged in
- A break-away tab to align the stencil. I'm not going to align this by hand, and a stencil is required to get reproducible results. If I didn't intend to make a lot of these, I'd just pre-tin all pads with a soldering iron.
- just place an LED somewhere near the highest current limiting resistor (4k3),
- rotate until it lights up,
- optionally: move to lower value resistors until it's bright enough.
The resistors are in 0201 packages to fulfill the actual purpose of this design: see if the moon phase has an effect on tombstoning. 4 land patterns are stock KiCad, 4 are a custom (hopefully better) land pattern.