LEDs, or Light Emmiting Diodes, are commonly used to indicate somthing about a system. The most simple use is to show that electrical power is present.

Lighting an LED takes three things:

• an LED, of course
• a source of electricity. Above, the schematic shows a battery, but any voltage source can be used, so long as we know what its voltage is.
• A resistor, to limit the ammount of current that flows.

We'll tackle each of these in turn.