Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Switching an LED on and off

This example brings together digital input and digital output. The program will read a button and turn the LED on and off according to the state of the button.

Before using the digital pins we need to set whether they are input or output. This is done via pinMode(context, 0, gInputPin, INPUT);.

You will notice that the LED will normally stay on and will turn off only when the button is pressed. This is due to the fact that the LED is set to the same value read at gInputPin. When the button is not pressed, gInputPin is HIGH and so gOutputPin is set to HIGH as well, so that the LED conducts and emits light. When the button is pressed, gInputPin goes LOW and gOutputPin is set to LOW, turning off the LED.

As an exercise try and change the code so that the LED only turns on when the button is pressed.

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The platform for ultra-low latency audio and sensor processing
A project of the Augmented Instruments Laboratory within the
Centre for Digital Music at Queen Mary University of London.
(c) 2016 Augmented Instruments Laboratory: Andrew McPherson,
Astrid Bin, Liam Donovan, Christian Heinrichs, Robert Jack,
Giulio Moro, Laurel Pardue, Victor Zappi. All rights reserved.
The Bela software is distributed under the GNU Lesser General Public License
(LGPL 3.0), available here:
#include <Bela.h>
#include <stdlib.h>
int gInputPin = 1; // digital pin 1 - check the pin diagram in the IDE
int gOutputPin = 0; // digital pin 0 - check the pin diagram in the IDE
bool setup(BelaContext *context, void *userData)
// Set the mode of digital pins
pinMode(context, 0, gInputPin, INPUT); //set input
pinMode(context, 0, gOutputPin, OUTPUT); // setoutput
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n=0; n<context->digitalFrames; n++){
int status=digitalRead(context, 0, gInputPin); //read the value of the button
digitalWriteOnce(context, n, gOutputPin, status); //write the status to the LED
float out = 0.1f * status * rand() / (float)RAND_MAX * 2.f - 1.f; //generate some noise, gated by the button
for(unsigned int j = 0; j < context->audioOutChannels; j++){
audioWrite(context, n, j, out); //write the audio output
void cleanup(BelaContext *context, void *userData)
// Nothing to do here