ABSTRACT
Another way to tell cathode from anode is to look at the rim (if the LED has one). The rim is flat on the side nearer the cathode wire.
An LED needs about 20 mA to light it to full brightness, though as little as 5 mA still produces a clearly seen glow. The forward voltage drop (p. 57) of an LED averages about 1.5 V, so a 2 V supply will light most types to their maximum brightness. When lit by a higher voltage, the LED may be burnt out if the forward voltage across it exceeeds 2 V. It is essential to wire a current limiting resistor in series with it.
