So, you found a circuit board with a bunch of varying color LEDs, but aren't sure what voltages and currents each needs. Or you just have a box with a bunch that are missing their labels because they were bought so long ago. Fear not! It is incredibly simple to identify them, and will lead to higher efficiency lights with proper resistor sizing, instead of just guessing or oversizing the resistor.
Honestly, this is making more of an issue of LEDs than it actually is. Most LEDs will require 20mA at a standard voltage based on their color. But items like superbright LEDs or COB (Chip on Board) LEDs can require significantly more current, and they are the most fun.
Step 1: Approximating the Voltage Drop
Based on the color of your led, you can use this chart:
or anything similar, to find what voltage level you are expecting. Most LEDs will also require the current specified in the chart, and that is all that is required.
But some LEDs are special, and will require higher amperages. At this point you can either set a voltage supply to nice number, like (1+the LED voltage drop), and use a potentiometer in series, slowly decreasing the resistance until a multimeter reads the correct voltage across the led, and either measure the current or the resistance of the potentiometer, calculating out current from the simple Ohm's law relationship of I=V/R, or in the case of our potentiometer, I=1/R, because we set the voltage to that handy (1+the LED voltage drop).
Or, if you have a constant current power supply, you can slowly bring the current up until the led lights up, and read the voltage. The current method works slightly better if like me, you are lazy and don't want to get out a potentiometer and multimeter. For instance, in the case of this small red led, at the recommended 1.8V it will draw 300mA, and be the same brightness as when it is 1.5V 20mA. From this we know that we should treat the LED as a 1.8V 20mA diode. This LED was used as an example because I knew that it is specced to run at 1.8V 20mA, but heats up very easily when given 1.8V with no current limit, which made it a convenient example.
Step 2: Testing
Using the second method, I tested two unknown LEDs, a green salvaged from a VCR player and a superbright red.
The green ended up slightly higher than the charts recommended 2.1V 20mA, but close enough that it confirms the green is indeed a 2.1V 20mA diode, as increasing the current did not make it any brighter. (Be careful when increasing the current like this. Any excess current will be turned into heat and could burn out the LED. The reason I showed the small red with such a high current draw was just to show that it would draw more than it needed. If I left that LED on for a while, it would burn out.)
The red I set to 2.1V because it was likely a "super red". At 2.1V it maxed out drawing 37mA, which tells me that the LED was designed to draw 40mA at 2.2V.
See how easy it is to reuse LEDs even if you don't have their official ratings? Most of the time testing them is not even necessary! I hope you enjoyed this example of how I generally start when reusing LEDs. I would be happy to answer any questions anyone has. If you liked this, remember to vote.