How does an LED work, and how do you tell which leg goes to positive?

An LED is a diode that emits light when current flows through it in one direction, from anode to cathode. The anode (+) is the longer leg; the cathode (−) is the shorter leg on the flat edge of the rim.

A diode is a one-way valve for current. A light-emitting diode is one built from a semiconductor that releases the energy of that current as light, and the colour depends on the semiconductor material. That has two practical consequences:

• Polarity matters. Wired the right way round (forward-biased), current flows and it lights. Reversed, almost no current flows and it stays dark. No current, no light.
• You have to identify the legs. The anode (+) is the longer leg. The cathode (−) is the shorter leg, and the plastic rim has a flat edge on that side. In the schematic symbol the triangle points in the direction of current, into the bar, which is the cathode.

LEDs come in many shapes and sizes, and the colour of the plastic lens is usually the colour of the light. Inside the lens are the semiconductor die, a wire bond, a reflective cavity and the two lead frames (anvil and post).

Tip: "Long leg = pLus". If the legs have been trimmed, look for the flat edge: flat = minus.

Go deeper:

• SparkFun — Light-Emitting Diodes — polarity, forward voltage by colour, and how to calculate the current-limiting resistor.