SparkFun Inventor's Kit Experiment Guide - v4.0

Contributors: Joel_E_B

Circuit 1C: Photoresistor

In circuit 1B, you got to use a potentiometer, which varies resistance based on the twisting of a knob. In this circuit you’ll be using a photoresistor, which changes resistance based on how much light the sensor receives. Using this sensor you can make a simple night-light that turns on when the room gets dark and turns off when it is bright.

Parts Needed

Grab the following quantities of each part listed to build this circuit:

New Components

Photoresistor

Photoresistors, or photocells, are light-sensitive, variable resistors. As more light shines on the sensor’s head, the resistance between its two terminals decreases. They’re an easy-to-use component in projects that require ambient-light sensing.

New Concepts

Analog to Digital Conversion

The world we live in is analog, but the RedBoard lives in a digital world. In order to have the RedBoard sense analog signals, we must first pass them through an Analog to Digital Converter (or ADC). The six analog inputs (A0--A5) covered in the last circuit all use an ADC. These pins "sample" the analog signal and create a digital signal for the microcontroller to interpret. The "resolution" of this signal is based on the resolution of the ADC. In the case of the RedBoard, that resolution is 10-bit. With a 10-bit ADC, we get 2 ^ 10 = 1024 possible values, which is why the analog signal varies between 0 and 1023.

Voltage Divider Continued

Since the RedBoard can’t directly interpret resistance (rather, it reads voltage), we need to use a voltage divider to use our photoresistor, a part that doesn't output voltage. The resistance of the photoresistor changes as it gets darker or lighter. That changes the amount of voltage that is read on the analog pin, which "divides" the voltage, 5V in this case. That divided voltage is then read on the analog to digital converter.

The voltage divider equation assumes that you know three values of the above circuit: the input voltage (V_in), and both resistor values (R₁ and R₂). Given those values, we can use this equation to find the output voltage (V_out):

If R₁ is a constant value (the resistor) and R₂ fluctuates (the photoresistor), the amount of voltage measured on the V_out pin will also fluctuate.

Hardware Hookup

Note that the photoresistor is not polarized. It can be inserted in either direction.

Ready to start hooking everything up? Check out the circuit diagram and hookup table below to see how everything is connected.

Circuit Diagram

Note for Advanced Users: If you know how to read datasheets and schematics, you can also refer to the schematic below as an alternative.

Having a hard time seeing the circuit? Click on the image for a closer look.

Hookup Table

Open the Sketch

To open the code, go to: File > Examples > SIK_Guide_Code-V_4 > SIK_Circuit_1C-Photoresistor

You can also copy and paste the following code into the Arduino IDE. Hit upload, and see what happens!

language:cpp
/*
  SparkFun Inventor’s Kit
  Circuit 1C-Photoresistor

  Use a photoresistor to monitor how bright a room is, and turn an LED on when it gets dark.

  This sketch was written by SparkFun Electronics, with lots of help from the Arduino community.
  This code is completely free for any use.

  View circuit diagram and instructions at: https://learn.sparkfun.com/tutorials/sparkfun-inventors-kit-experiment-guide---v40
  Download drawings and code at: https://github.com/sparkfun/SIK-Guide-Code
*/

int photoresistor = 0;              //this variable will hold a value based on the brightness of the ambient light
int threshold = 750;                //if the photoresistor reading is below this value the the light will turn on

void setup()
{
  Serial.begin(9600);               //start a serial connection with the computer

  pinMode(13, OUTPUT);              //set pin 13 as an output that can be set to HIGH or LOW
}

void loop()
{
  //read the brightness of the ambient light
  photoresistor = analogRead(A0);   //set photoresistor to a number between 0 and 1023 based on how bright the ambient light is
  Serial.println(photoresistor);    //print the value of photoresistor in the serial monitor on the computer

  //if the photoresistor value is below the threshold turn the light on, otherwise turn it off
  if (photoresistor < threshold) {
    digitalWrite(13, HIGH);         // Turn on the LED
  } else {
    digitalWrite(13, LOW);          // Turn off the LED
  }

  delay(100);                       //short delay to make the printout easier to read
}

What You Should See

The program stores the light level in a variable, photoresistor. Then, using an if/else statement, the program checks to see what it should do with the LED. If the variable is above the threshold (it’s bright), turn the LED off. If the variable is below the threshold (it’s dark), turn the LED on. You now have just built your own night-light!

Open the Serial Monitor in Arduino. The value of the photoresistor should be printed every so often. When the photoresistor value drops below the threshold value set in the code, the LED should turn on (you can cover the photoresistor with your finger to make the value drop).

Program Overview

1. Store the light level in the variable photoresistor.
2. If the value of photoresistor is above the threshold (it’s bright), turn the LED off.
3. If the value of photoresistor is below the threshold (it’s dark), turn the LED on.

Code to Note

Coding Challenges

Troubleshooting