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LED w/Resistor

Overview​

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 0V. Let's examine a typical single resistor circuit:

Diagram showing a circuit with a source voltage (V S), resistor (R) and LED with voltage drop (V F) and current (I).

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:

Circuit diagram showing a Netduino board connected on a breadboard to an LED through a resistor to pin 9.

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
            }
        }
    }
}

Netduino.Foundation Support​

Netduino.Foundation includes a number of classes that make driving LEDs with PWM signals easy. They include the ability to control the PWM by setting the brightness, and even have built-in animations.

When using a resistor with one of these classes, pass TypicalForwardVoltage.ResistorLimited or 0 to the forwardVoltage parameter in the constructor.

PwmLed Class​

The PwmLed class represents an LED whose voltage (and brightness) is controlled by the duty-cycle of a PWM signal. Can be used both with LEDs that have been current limited with in-series resistors, or LEDs without resistors.

RgbPwmLed Class​

The RgbPwmLed class represents an RGB LED whose color is controlled by the duty-cycle of three PWM signals. Can be used both with LEDs that have been current limited with in-series resistors, or LEDs without resistors.