Lesson 22 – Photo Resistor


1. Introduction
In the lesson, we will connect Photo Resistor to Keyestudio RPI GPIO-PCF8591 Shield to test the extended AD function of raspberry pi. We will directly connect an external photo resistor to test the illumination intensity, and then use the measured value to control LED’s on and off.

2. Hardware Required

  • Photo Resistor*1
  • LED – Red*1
  • 10KΩ Resistor*1
  • 220Ω Resistor*1
  • RPI GPIO Shield*1
  • 40Pin Colorful Ribbon Cable *1
  • Keyestudio RPI GPIO-PCF8591 Shield*1
  • Breadboard*1
  • USB Cable*1
  • Raspberry Motherboard*1
  • Breadboard Wires* Several
  • Dupont Wire* Several

3. Connection Diagram

4. Programming

  • a. Raspberry pi default not open the I2C function, on the terminal, input sudo raspi-config to open raspberry configuration interface.
    b. Select the 7 Advanced Options, then select the A7 I2C option to enable the raspberry pi I2C function.
    c. Use winSCP to put the lesson21_Photo_Resistor into the lesson folder of raspberry system.
    d. In the terminal, input cd lesson and cd lesson21_Photo_Resistor to enter the folder.
    e. In the terminal, execute make to generate a Photo_Resistor executable file inside the folder.
    f. Wiring as the above diagram, then enter the lesson21_Photo_Resistor folder, execute sudo ./Photo_Resistor at the terminal. Then, you can see the value(0-255) representing the illumination intensity; when the value is greater than 150, LED lights up.
    g. Ctrl + c can exit the processing program.Please refer to the figure below:

5. Sample Code





The first four lines below are –

  • #include <wiringPi.h>
  • #include <pcf8591.h>
  • #include <stdio.h>
  • #include <stdlib.h>

#define Address 0x48
#define BASE 64
#define A0 BASE+0
#define A1 BASE+1
#define A2 BASE+2
#define A3 BASE+3

int main(void)
      unsigned char value;



  1. This development track is based on the Rasbperry Pi and the Ultimate Starter Kit for the Raspberry Pi.
  2. You will need access to both the Raspberry Pi SBC and the electronics components part of the Ultimate Starter Kit for the Raspberry Pi kit to be able to work on these tutorials.
  3. If you haven’t purchased the Raspberry Pi 4 B yet please head over to our store and purchase one now. You can pick up the Ultimate Starter Kit for the Raspberry Pi from OztoyLib.
  4. Depending on where you live you might also be able to pick up the Raspberry Pi and Ultimate Starter Kit for the Raspberry Pi at your local electronics hobby store.

About the Ultimate Starter Kit for the Raspberry Pi

The Ultimate Starter Kit for the Raspberry Pi comes packed with ~37 different electronic bits (Sensors, LEDs, switches, LCD, servo, etc.) including tutorials to get started. You will learn how to connect up the ~37 different electronic bits (Sensors, LEDs, switches, LCD, servo, etc.), create circuits using the Raspberry Pi including learning to write code in C. As you progress through the different tutorials you will explore the different capabilities of the Raspberry Pi including the intricacies of integrating the Raspberry Pi with the different electronic bits i.e. sensors, LED’s, switches, servos, etc. included in the starter kit.

The Ultimate Starter Kit for the Raspberry Pi comes along with detailed tutorials including code samples. Ultimate Starter Kit for the Raspberry Pi walks you through the basics of using the Raspberry Pi in a hands-on way. You’ll learn the fundamentals of electronics, programming in C and gain hands on experience through working on the Raspberry Pi building creative projects. The kit includes a selection of the most common and useful electronic components with an ebook of 37 projects. Starting the basics of electronics, to more complex projects, the kit will get you interacting with the physical world using sensor and actuators. Along with the kit you get access to detailed tutorials and wiring diagrams.

About the Raspberry Pi

The Raspberry Pi is a series of small single-board computers developed in the United Kingdom by the Raspberry Pi Foundation to promote the teaching of basic computer science in schools and in developing countries. It is a capable little computer which can be used in electronics projects, and for many of the things that your desktop PC does, like spreadsheets, word processing, browsing the internet, and playing games. The original model became far more popular than anticipated, selling outside its target market for uses such as robotics.

The Raspberry Pi does not include peripherals (such as keyboards, mice and cases). However, some accessories have been included in several official and unofficial bundles. According to the Raspberry Pi Foundation, over 5 million Raspberry Pis were sold by February 2015, making it the best-selling British computer. By November 2016 they had sold 11 million units, and 12.5m by March 2017, making it the third best-selling “general purpose computer”. In July 2017, sales reached nearly 15 million.In March 2018, sales reached 19 million. Most Pis are made in a Sony factory in Pencoed, Wales; some are made in China or Japan.

You can read more about the Raspberry Pi here – RaspberryPi.org.