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2.2.1 Photoresistor(MCP3008)

Note

Depending on your kit version, please identify whether you have ADC0834 or MCP3008 and proceed with the matching section.

Introduction

Photoresistor is a commonly used component of ambient light intensity in life. It helps the controller to recognize day and night and realize light control functions such as night lamp. This project is very similar to potentiometer, and you might think it changing the voltage to sensing light.

Required Components

In this project, we need the following components.

Principle

A photoresistor or photocell is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photo conductivity. A photoresistor can be applied in light-sensitive detector circuits, and light- and darkness-activated switching circuits.

Schematic Diagram

T-Board Name	physical	WiringPi	BCM
SPICE0	pin24	10	8
SPIMOSI	pin19	12	10
SPIMISO	pin21	13	9
SPISCLK	pin23	14	11
GPIO22	pin15	3	22

Experimental Procedures

Step 1: Build the circuit.

Step 2: Go to the folder of the code.

cd ~/davinci-kit-for-raspberry-pi/c/2.2.1-2/

Step 3: Compile the code.

gcc 2.2.1_Photoresistor.c -o photoresistor -lwiringPi -lm

Step 4: Run the executable file.

./photoresistor

When the code is running, the brightness of the LED will change according to the light intensity sensed by the photoresistor.

Note

If it does not work after running, or there is an error prompt: "wiringPi.h: No such file or directory", please refer to Install and Check the WiringPi.

Code

#include <wiringPi.h>
#include <wiringPiSPI.h>
#include <stdio.h>
#include <softPwm.h>

#define SPI_CHANNEL 0      // Use SPI channel 0 (CE0)
#define SPI_SPEED   1000000 // 1 MHz SPI speed
#define LedPin      3       // GPIO3 (WiringPi) for LED PWM

// Read ADC value from MCP3008, channel 0~7
int readMCP3008(int channel) {
    if (channel < 0 || channel > 7) return -1;

    unsigned char buffer[3];
    buffer[0] = 1;                          // Start bit
    buffer[1] = (8 + channel) << 4;         // SGL/DIF = 1, D2-D0 = channel
    buffer[2] = 0;

    wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

    // Combine the result
    int result = ((buffer[1] & 3) << 8) | buffer[2];
    return result;
}

int main(void) {
    if (wiringPiSetup() == -1) {
        printf("wiringPi init failed!\n");
        return 1;
    }

    if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
        printf("SPI setup failed!\n");
        return 1;
    }

    softPwmCreate(LedPin, 0, 100); // Init software PWM

    while (1) {
        int analogVal = readMCP3008(0); // Read from CH0
        printf("ADC Value: %d\n", analogVal);

        // Scale 10-bit ADC value (0–1023) to PWM range (0–100)
        int pwmVal = analogVal * 100 / 1023;
        softPwmWrite(LedPin, pwmVal);

        delay(100);
    }

    return 0;
}

Code Explanation

The codes here are the same as that in 2.1.4 Potentiometer. If you have any other questions, please check the code explanation of 2.1.4 Potentiometer(MCP3008) for details.