Versatile LED Tester with Eagle Project Files
Eagle project files for the Muffsy Constant Current LED Tester. This is the preferred version, as it has a power on LED that acts as a load for the LM317 so that it can deliver lower currents.
x-zip-compressed - 32.30 kB - 11/29/2016 at 22:06
Eagle project files for the Muffsy Constant Current LED Tester. It's not recommended to use this project, since it will only give a minimum of ~2-3 mA.
x-zip-compressed - 22.32 kB - 11/28/2016 at 12:17
The discussion in the previous log showed that I'll never get less than 2-3 mA from an LM317, as that is the minimum current it can provide while still regulating. (Discussion here: https://hackaday.io/project/18624-muffsy-constant-current-led-tester/log/49640-testing-my-leds#discussion-list)
I looked for a way to make sure there was a current draw, while still allowing me to adjust the current to the "LUT" (LED Under Test), and found this: http://electronics.stackexchange.com/questions/211249/lm317-µa-constant-current-source-possibility
An LED from LM317's power output to ground acts as a load and draws current from the voltage regulator. Some 4 mA would be sufficient.
Here's the updated schematic:
Here's a PCB with the new circuit, the Eagle files have been replaced with this version:
The pictures didn't come out particularly well, as the highest brightness gave a lot of glare and the lowest brightness seems brighter than it really is.
Lowest brightness (it's barely visible in real life):
Highest brightness (pretty darn bright):
LEDs keep accumulating, and they are spread around in my work space. Most of them are clear, see-through LEDs. Some are dead (either by abuse, or because they are cheap Chinese diodes).
Most of them are high intensity LEDs, and they often need below 1 mA so they won't blind me.
In other words:
I need a way to test my LEDs to see if they work, which color they are, and find a suitable brightness.
After having used the LM317 in various projects, I had noticed that it could be used as a constant current source. This got me thinking about using it with LEDs.
Here's the circuit I came up with:
Forget for a moment that the pinout on the regulator is wrong (the attached Eagle project files are correct). Here's how it works:
I built it, and it works.
The linear pot that I used makes the LED go darker relatively fast, a logarithmic one is better.