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Sensors-LDR

This example demonstrates how to hook up a Light Dependent Resistor (LDR), also know as a photo-resistor, to Bela 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.

  •  connect one leg of the LDR to 3.3V
  •  connect the other leg of the LDR to analog input 1
  •  connect one leg of a 10KOhm resistor to this leg of the LDR as well
  •  connect the other leg of the resistor to ground

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`).

License: GNU Lesser General Public License (LGPL 3.0)
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