LED data sheet specifications:
Voltage = 2 Vdc (drop)
Current = 15 mA
But we have a 5 Vdc supply...
R = (Vs - Vled) / Iled
R = (5 V - 2 V) / .015 A
R = 200 Ohms
Notice that as R ^, the LED gets dimmer
(looks off at ~ 2 K Ohms),
and as R decreases the LED gets brighter,
until we reach ~ 97.5 Ohms, at which point the LED "burns out". The voltage is still O.K., but the current is 30 mA which is 50% above the LED's spec of 20 mA.
