RPi Tutorial EGHS:LED output

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GPIO Hardware & Software Tutorials:


While most of these circuits may interface directly to the RPi, the use of a buffered interface (such as the one supplied by the Gertboard) is recommended which will help protect against damage. Alternatively, experiment with one of the Alternative Test Platforms.

Extreme caution should be exercised when interfacing hardware at a low level, you may damage your RPi, your equipment and potentially yourself and others. Doing so is at your own risk!


The purpose of this guide is to enable control of an LED via the GPIO pins of the RPi.

This is the embedded version of writing a program to display "Hello World" and is the first step in getting started.

The first stage will be to build the hardware we are going to use, and then we shall look at the software which will drive it.

Until RPi devices are available, I can not confirm this will work on a real RPi.
For now, I shall be using the TI LaunchPad (see  Alternative Test Platforms
for details) to test the hardware on (as it is cheap and the logic levels similar).

The Hardware


This is only a brief and rough overview, since the basics are covered in a lot more detail in many other places (see below).

The GPIO pins on the RPi when defined as an Output is able to cause the voltage on the pin to go HIGH (source) or LOW (sink). This allows signals to be sent to other processors and devices like LEDs. However it is important to remember that the pin will only be able to Source or Sink very small currents, so higher powered devices (such as motors) can not be driven directly from a GPIO pin.

Depending on the specification of the RPi GPIO pins, the current SOURCE ability may be better,
than the SINK (or vice-a-versa).
i.e. If the RPi is able to SINK more current than it can SOURCE, then any driving circuit should
     be between the RPi 3.3V pin and the GPIO pin (rather than GPIO pin and GND).

For additional detail see [Introduction To Embedded Programming - GPIO Output]

Circuit 1 - Basic LED Driving Circuit

Basic LED Output Circuit 1

The resistor R1 is used to limit the current going through the LED (which has hardly any resistance), without the resistor, the LED will draw as much current as it can until it burns out (or burns out your GPIO pin).

The value you select for R1 will depend on the current required by the LED (upto 20mA depending on the LED used - check the datasheet) and the source current limit of the GPIO (launchpad is ~20mA), the RPi has a 50mA limit for the 3.3V supply line.

We also need to know the forward voltage required by the LED to light, typically around 2V-3.5V depending on colour[1].

Finally, the output voltage of the RPi (and LaunchPad) GPIO is 3.3V output level.

Vout = 3.3V
Vled = 2V (I'm using RED)
Iled = 5mA = 0.005A
R1 = (Vout – Vled)/Iled
   = (3.3 - 2)/0.005
   = 260ohms (so 270ohms is closest preferred value)

If in doubt, use a bigger resistor (=less current & less brightness) and test if good enough by connecting across the 3.3V and ground pins (if you are just experimenting you are unlikely to need LEDs shining at their full brightness anyway).

For instance, one of my test circuits uses 470ohms (which only gives 2.7mA on 3.3V, but the same
circuit can be connected to a 12V supply without blowing the LED - rated @20mA).

Basic LEDx8 Test Module

Basic LEDx8 Test Module (Control pins at top, GND connection at bottom-right).

Basic LEDx8 Test Module
Circuit with track breaks marked in red

The above test module has been built to allow easy testing of GPIO outputs by driving up to 8 LEDs. The resistor value 330 ohms is used (keeps the current draw fairly low).

The Software

While the RPi is not available, I can only confirm the TI LaunchPad code works for me.

TI LaunchPad

Sample test code for Basic LEDx8 Test Module (tested on TI MSP430G2553 device).

Basic LEDx8 Test Module input pins 0-7 wired to device Port1:0 to Port1:7, plus GND connection.


  • main.c - Main calling functions


The above circuits should work with code similar to that given in (RPi Low-level peripherals#Code examples) section.


  1. Wikipedia:LED Forward Voltages