Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
 All Classes Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups

Using light as a controller

This example demonstrates how to hook up a Light Dependent Resistor (LDR), also know as a photo-resistor, and use it to control the volume of white noise. If you run this project straight away after connecting up the LDR you will notice that the LDR reacts to changes in the light condition but in a pretty unsatisfying way. This is because we need to set the thresholds of the map() function based on the ambient light condition.

To begin let's connect the LDR.

The resistance of the LDR changes depending on the amount of light it receives (more light -> less resistance). To measure this change in resistance we need a fixed value resistor (10kOhms in this example) to compare the reading with. This is know as a voltage divider circuit. The 3.3V will be shared between the two resistors: how much of a share of voltage each of the resistors take is proportional to their resistances. As the resistance of the LDR changes the amount of voltage on each resistor changes and we can measure this change to tell how much light the LDR is receiving.

In order to use the LDR as a volume control we need to set the thresholds for ambient light and for when it has a bright light shone close to it. To do this comment out this section of code in render():

```` if(!n%2){ if(gSampleCount >= 44100) { rt_printf("%f\n", analogRead(context, n, 1)); gSampleCount = 0; } } gSampleCount++; ````

This prints the value of the LDR reading to the console once a second. Now you can set the variables gDark and gLight with the reading of ambient light in the room and with the reading when a torch is shone directly at the LDR. Update the variables, re-comment out the code and run the example. Now you should be able to bring the white noise from silence to full volume depending on the amount light.

Note that when you cover the LDR you should also hear an increase in volume of the white noise. This is because the map() function is not constrained which means that it outputs negative number when it gets darker than the threshold set in gDark. To stop this behaviour you can use the constrain() function to force the LDR readings to remain within a certain range (for example gDark to gLight).

____ _____ _ _
| __ )| ____| | / \
| _ \| _| | | / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/ \_\
#include <Bela.h>
#include <stdlib.h>
float gLDR;
float gNoise;
// replace these values with thresholds for dark and light
float gDark = 0.0;
float gLight = 1.0;
// sample counter to print current LDR reading
int gSampleCount = 0;
int gAudioFramesPerAnalogFrame = 0;
bool setup(BelaContext *context, void *userData)
fprintf(stderr, "Error: This example requires analog inputs to be enabled");
return false;
if(context->analogSampleRate > context->audioSampleRate)
fprintf(stderr, "Error: for this project the sampling rate of the analog inputs has to be <= the audio sample rate\n");
return false;
gAudioFramesPerAnalogFrame = context->audioFrames / context->analogFrames;
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->audioFrames; n++) {
if(gAudioFramesPerAnalogFrame && !(n % gAudioFramesPerAnalogFrame)) {
// read and map the LDR
gLDR = map(analogRead(context, n, 1), gDark, gLight, 0.0, 0.5);
// Uncomment the below code to print the LDR reading to the console
// to set the thresholds
if(gSampleCount >= 44100) {
rt_printf("%f\n", analogRead(context, n, 1));
gSampleCount = 0;
// generate some noise with random number generator
gNoise = 0.1f * ( rand() / (float)RAND_MAX * 2.f - 1.f);
for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
// output the noise multiplied by the LDR reading
audioWrite(context, n, channel, gNoise*gLDR);
void cleanup(BelaContext *context, void *userData)