By David Somerset | Updated Mar 24, 2022
A diode is an electronic semiconductor that allows current to flow in only one direction. A light‑emitting diode (LED) illuminates when current flows correctly. Modern LEDs cover the full visible spectrum, making them versatile for countless applications. To control when the LED lights, you need to wire it to a switch.
Check the rated voltage and current of the LED you purchased.
Determine the required series resistor with the formula: (source voltage – LED voltage drop) / LED current = resistance in ohms. For example, with a 12‑V supply, a 3.1‑V LED, and 20 mA current, the calculation is (12 – 3.1) / 0.020 ≈ 445 Ω. Buy a resistor of the nearest standard value, rounding up if necessary.
Solder the resistor to the LED’s anode (the longer lead).
With the switch in the “Off” position, connect one end of the resistor to one terminal of the switch using copper wire. Solder both connections securely.
Solder a wire from the other terminal of the switch to the positive (+) terminal of the power supply (typically the red lead).
Connect a third wire from the LED’s cathode (shorter lead) to the negative side of the power supply. Solder all joints firmly.
Flip the switch to “On” and confirm the LED lights as expected.
Properly connect the LED’s anode and cathode, include the correct resistor, and wire through a switch to ensure safe, reliable operation.
Always use the appropriate resistor value. Excess current can damage the LED, potentially causing it to fail catastrophically. Adhering to the calculated resistance protects both the component and the user.
