Everyone who works with LED’s need to know what colour is the LED, how much current it needs to shine the best, what resistor is needed.
David Cook has designed a simple to make and use LED tester which is powered from 9V battery. Tester is made out of easy accessible parts. It is based on adjusting the current from 2mA to 26mA, the voltage adjusts automatically depending on voltage needed by the LED. It has several test points for measuring the current and voltage of the led. Also this LED tester’s circuit is protected against short. [link]