Those brilliant ideas you get in the middle of the night are often forgotten. And those that aren't forgotten are generally crap anyway.
I remembered this one, and it turned into a small project:
If a voltage regulator always keeps the a constant voltage between the regulator and adjust pins, why not use this as a constant current source?
The idea is not new in any way or form (see page 20), but it had never occurred to me before. But when it did, I immediately thought of testing LEDs. I have quite a lot of them. They are transparent, so I can't really tell which color they are. And many eBay/Aliexpress LEDs are dead on arrival, or broken by abuse. And they are always too damn bright.
This constant current LED tester shows the color of my LEDs, if they are working, and lets me change the brightness. It can also determine current and forward voltage drop at that brightness.
That last part is quite important, since the forward voltage drop decreases at lower brightness (sometimes more than one volt!), which results in more light from the diode than you had expected.
It might seem odd to add a power on LED to an LED tester, but it's there for a reason. The minimum current a voltage regulator can provide is ~2 mA. So that extra LED is there to draw current from the LM317 and let the constant current circuit deliver less than those 2 mA.
(Make Your Own Muffsy Constant Current LED Tester)
My needs are covered by this LED Tester, but there are always other itches to scratch. This project has been around long enough to give theslowdiyer time to make his own deluxe version:
If you'd like to build the Muffsy Constant Current LED Tester, or modify it before doing so, the Eagle project files are freely available.
Use them in any way you desire, personal use, commercial use, group buys, as part of another project, I don't mind. I won't even ask that you give me any credit for it. The only condition is that you don't blame me if something goes wrong in a bad way.