Photo Resistors

Whilst getting input from a potentiometer can be useful for human controlled experiments, what do we use when we want an environmentally controlled experiment? We use exactly the same principles but instead of a potentiometer (twist based resistance) we use a photo resistor (light based resistance). The Arduino cannot directly sense resistance (it senses voltage) so we set up a voltage divider ( The exact voltage at the sensing pin is calculable, but for our purposes (just sensing relative light) we can experiment with the values and see what works for us. A low value will occur when the sensor is well lit while a high value will occur when it is in darkness.


  • 2 pin header x 4
  • 5mm LED
  • 330 ohm resistor (orange-orange-brown)
  • 10k Ohm Resistor (brown-black-orange)
  • Photo resistor
  • jumper wires

Circuit Layout

Circuit Layout

Circuit Assembly

Assembly Diagram

Assembly video:


You can find this code in code/CIRC-09-code-photoresistor.js

var five = require("johnny-five"),
    board, myPhotoresistor, myLed;
board = new five.Board();
board.on("ready", function() {
  myLed = new five.Led(9);
  myPhotoresistor = new five.Sensor({
    pin: "A0",
    freq: 250
  myPhotoresistor.on("read", function( err, value ) {
    // range of led brightness is 0 - 255
    var brightnessValue = five.Fn.constrain(, 0, 900, 0, 255), 0, 255);


LED Remains Dark

This is a mistake we continue to make time and time again, if only they could make an LED that worked both ways. Pull it up and rotate it.

It Isn't Responding to Changes in Light.

Given that the spacing of the wires on the photo-resistor is not standard, it is easy to misplace it. Double check it's in the right place.

Still not quite working?

You may be in a room which is either too bright or dark. Try turning the lights on or off to see if this helps. Or if you have a flashlight near by give that a try.

Extending the Code

Reverse the response:

Perhaps you would like the opposite response. Don't worry we can easily reverse this response just add:

brightnessValue = 255 - brightnessValue;

Run the program again and watch the response change

Night light:

Rather than controlling the brightness of the LED in response to light, let's instead turn it on or off based on a threshold value. Change the photoresistor's data handler to:

myPhotoresistor.on("data", function( err, value ) {
  var threshold = 300;
  if (value > threshold) {
  } else {;


myPhotoresistor.booleanAt(512).on("data", function( err, value ) {
    if (this.boolean) {
    } else {;

Light controlled servo

Let's use our newly found light sensing skills to control a servo. Wire up a servo connected to pin 9 (like in Exercise 4). Then open the code you wrote for controlling a servo in Exercise 8 and satch as it works unmodified.

Using the full range of your servo:

You'll notice that the servo will only operate over a limited portion of its range. This is because with the voltage dividing circuit we use the voltage on analog pin 0 will not range from 0 to 5 volts but instead between two lesser values (these values will change based on your setup). To fix this play with the five.Fn.constrain(, 0, 1023, 0, 179), 0, 255); line.


For more details on this circuit, see