Fading LEDs

This sketch uses a sine wave to drive the brightness of a series of LEDs connected to the eight analog out pins. Again you can see the nested for loop structure but this time for the analog output channels rather than the audio.

  • connect an LED in series with a 470ohm resistor between each of the analogOut pins and ground.

Within the first for loop in render we cycle through each frame in the analog output matrix. At each frame we then cycle through the analog output channels with another for loop and set the output voltage according to the phase of a sine tone that acts as an LFO. The analog output pins can provide a voltage of ~4.092V.

The output on each pin is set with analogWrite() within the for loop that cycles through the analog output channels. This needs to be provided with arguments as follows analogWrite(context, n, channel, out). Channel is where the you give the address of the analog output pin (in this case we cycle through each pin address in the for loop), out is the variable that holds the desired output (in this case set by the sine wave) and n is the frame number.

Notice that the phase of the brightness cycle for each led is different. This is achieved by updating a variable that stores a relative phase value. This variable is advanced by pi/4 (1/8 of a full rotation) for each channel giving each of the eight LEDs a different phase.

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| |_) | |___| |___ / ___ \
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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 <cmath>
// Set range for analog outputs designed for driving LEDs
const float kMinimumAmplitude = (1.5 / 5.0);
const float kAmplitudeRange = 1.0 - kMinimumAmplitude;
float gFrequency = 3.0;
float gPhase;
float gInverseSampleRate;
bool setup(BelaContext *context, void *userData)
// Check if analog channels are enabled
if(context->analogFrames == 0 || context->analogFrames > context->audioFrames) {
rt_printf("Error: this example needs analog enabled, with 4 or 8 channels\n");
return false;
// Check that we have the same number of inputs and outputs.
if(context->audioInChannels != context->audioOutChannels ||
context->analogInChannels != context-> analogOutChannels){
printf("Error: for this project, you need the same number of input and output channels.\n");
return false;
gInverseSampleRate = 1.0 / context->analogSampleRate;
gPhase = 0.0;
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->analogFrames; n++) {
// Set LED to different phase for each matrix channel
float relativePhase = 0.0;
for(unsigned int channel = 0; channel < context->analogOutChannels; channel++) {
float out = kMinimumAmplitude + kAmplitudeRange * 0.5f * (1.0f + sinf(gPhase + relativePhase));
analogWrite(context, n, channel, out);
// Advance by pi/4 (1/8 of a full rotation) for each channel
relativePhase += M_PI * 0.25;
// Update and wrap phase of sine tone
gPhase += 2.0 * M_PI * gFrequency * gInverseSampleRate;
if(gPhase > 2.0 * M_PI)
gPhase -= 2.0 * M_PI;
void cleanup(BelaContext *context, void *userData)