Adding LEDs to projects - calculating series resistors.

[ppppenguin]

If the supply voltage is very close to the LED voltage (happens more often if you have a string of LEDs in series) then you end up with a very low resistor value. This gives a variable and rather unpredictable current as the LED voltage drops vary with temperature. As a very rough and ready rule if the drop across the resistor is less than the drop across the LED(s) be careful.

If you connect a LED directly to a voltage supply such as a battery then that's asking for trouble. The current will be utterly unpredictable.