This page explains the most basic circuit for lighting up an LED - and how to calculate the value of current-limiting resistor that is needed.

======Basic Circuit======

{{ :led-circuit-basic.png |}}

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.

======batteries and other voltage sources======

======LEDs======

LED is short for Light Emitting Diode.  The light-emitting part is self-explanatory.  "Diode" requires explanation.  "Diode" literally means an somthing with two terminals, or nodes.  They have the property that they only allow electricity to flow in one direction, hence the big arrow in their schematic symbol. 

An "ideal diode" would conduct electricity perfectly, without any loss, in the direction of the arrow, and perfectly block any current attempting to flow in the other direction.
In contrast, any practical diode in the real world differs from that ideal in ways that we have to consider.

=====Diode forward voltage=====

A key property of any diode is a number called its forward voltage, Vf. Applying a voltage that is of the correct polarity, but less than the forward voltage, will still not cause any current to flow.  Imagine water blocked behind a dam, but lower than the height of the dam: no water flows out of the lake.

As the voltage is increased to the forward voltage, current begins to flow, and our LED starts to light up.  Picture water just dribbling over the top of a dam.

Trying to apply a voltage much larger than the forward voltage will cause a very large ammount of current to flow.  Continuing our water analogy: a massive torrent flooding over the dam.
This analogy also suggests why we need a resistor: to limit, or resist, the ammount of current that flows.

For a whole lot more detail on LEDs, including the physics of how they produce light, see the wikipedia page: https://en.wikipedia.org/wiki/Light-emitting_diode



======Resistors======

======Calculating the resistor value for our LED circuit======