To drive an LED with a resistor as the current limiting device in the circuit, we need to first compute the resistance needed.

Calculating Necessary Resistance

Kirchoff’s Voltage Law states that the sum of the voltages in any circuit loop (source -> load -> return/common) must be zero. Which means that if the source is 5V, then the total load must drop 5V to zero. Let’s examine a typical single resistor circuit:

So to calculate the resistance needed in this circuit to power the LED, we use Ohm’s law, solved for resistance, but remove the voltage drop specified as the forward voltage (Fv) from the source voltage (Vs) for the LED we’re using:

R = (Vs - Fv) / I

Example

For example, let’s say that we have a red LED has a maximum current rating of 20mA, and a Vf of 1.8V, that we’re driving from a 5V voltage source. Solving for R:

R = (5V - 1.8V) / 0.020A = 160Ω 

The circuit would need at least a 160Ω resistor to safely drive the LED. Note that when calculating the resistance, 20mA was converted to 0.020A.

Online Calculator

To aid in LED resistance calculation, here is a fantastic online LED resistance calculator.

Sample Project

Prototyping this with a Netduino would look something like this:

The following code can then be used to make that LED blink by repeatedly turning it on and off:

using System;
using Microsoft.SPOT;
using Microsoft.SPOT.Hardware;
using SecretLabs.NETMF.Hardware.Netduino;
using System.Threading;

namespace Blinky
{
	public class Program
	{
		public static void Main()
		{
			// Create a new output port on Digital Pin 7
			OutputPort led = new OutputPort(Pins.GPIO_PIN_D7, false);
			while (true)
			{
				led.Write(true); // turn on the LED
				Thread.Sleep(250); // sleep for 250ms
				led.Write(false); // turn off the LED
				Thread.Sleep(250); // sleep for 250ms
			}
		} 
	}
}