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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

_images/schematic_2.2.1_photoresistor_mcp3008.png

Experimental Procedures

Step 1: Build the circuit.

_images/july24_2.2.1_photoresistor_mcp3008.png

For C Language Users

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.

For Python Language Users

Step 2: Set up the SPI interface and install the spidev library (see SPI Configuration for detailed instructions). If you have already completed these steps, you can skip this.

Step 3: Go to the folder of the code.

cd ~/davinci-kit-for-raspberry-pi/python

Step 4: Run the executable file.

sudo python3 2.2.1-2_photoresistor.py

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

Warning

If there is an error prompt RuntimeError: Cannot determine SOC peripheral base address, please refer to If gpiozero doesn’t work.

Code

Note

You can Modify/Reset/Copy/Run/Stop the code below. But before that, you need to go to source code path like davinci-kit-for-raspberry-pi/python. After modifying the code, you can run it directly to see the effect.

#!/usr/bin/env python3

import RPi.GPIO as GPIO
import spidev
import time

# GPIO pin for PWM LED
PWM_PIN = 22

# Setup GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(PWM_PIN, GPIO.OUT)

# Initialize PWM (frequency = 1000Hz)
pwm = GPIO.PWM(PWM_PIN, 1000)
pwm.start(0)  # Start with 0% duty cycle

# Initialize SPI (MCP3008 on Bus 0, CE0)
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1000000  # 1 MHz

# Function to read MCP3008 ADC value
def read_adc(channel):
    """
    Read analog value from MCP3008 (channel 0–7)
    Returns: 10-bit value (0–1023)
    """
    if channel < 0 or channel > 7:
        return -1
    r = spi.xfer2([1, (8 + channel) << 4, 0])
    value = ((r[1] & 3) << 8) | r[2]
    return value

# Main loop to read ADC and set PWM brightness
try:
    while True:
        analogVal = read_adc(0)
        print(f"value = {analogVal}")

        # Scale ADC value (0–1023) to duty cycle (0–100)
        duty_cycle = analogVal * 100 / 1023
        pwm.ChangeDutyCycle(duty_cycle)

        time.sleep(0.2)

except KeyboardInterrupt:
    pass

finally:
    pwm.stop()
    GPIO.cleanup()
    spi.close()

Code Explanation

Import necessary libraries:
- RPi.GPIO to control GPIO pins and generate PWM signal.
- spidev to interface with the MCP3008 ADC via SPI.
- time to handle timing and delays.
```
#!/usr/bin/env python3

import RPi.GPIO as GPIO
import spidev
import time
```

Configure GPIO pin 22 as PWM output using BCM mode. Then, initialize software PWM at 1000 Hz with a starting duty cycle of 0%.

# GPIO pin for PWM LED
PWM_PIN = 22

# Setup GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(PWM_PIN, GPIO.OUT)

# Initialize PWM (frequency = 1000Hz)
pwm = GPIO.PWM(PWM_PIN, 1000)
pwm.start(0)  # Start with 0% duty cycle

Set up the SPI interface to communicate with MCP3008 on bus 0, chip enable 0 (CE0), and configure SPI speed to 1 MHz.

# Initialize SPI (MCP3008 on Bus 0, CE0)
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 1000000  # 1 MHz

Define a function read_adc(channel) to read analog values from MCP3008. The function sends three bytes to the chip and reconstructs a 10-bit analog value (0–1023) from the response.

# Function to read MCP3008 ADC value
def read_adc(channel):
    """
    Read analog value from MCP3008 (channel 0–7)
    Returns: 10-bit value (0–1023)
    """
    if channel < 0 or channel > 7:
        return -1
    r = spi.xfer2([1, (8 + channel) << 4, 0])
    value = ((r[1] & 3) << 8) | r[2]
    return value

This is the main loop that:

Reads analog input from channel 0 of the MCP3008.
Converts the value to a PWM duty cycle (0–100%).
Adjusts the brightness of the LED using pwm.ChangeDutyCycle().
Repeats every 0.2 seconds.

# Main loop to read ADC and set PWM brightness
try:
    while True:
        analogVal = read_adc(0)
        print(f"value = {analogVal}")

        # Scale ADC value (0–1023) to duty cycle (0–100)
        duty_cycle = analogVal * 100 / 1023
        pwm.ChangeDutyCycle(duty_cycle)

        time.sleep(0.2)

When the user interrupts the program with Ctrl+C, the PWM and GPIO are properly cleaned up, and the SPI interface is closed.
```
except KeyboardInterrupt:
    pass

finally:
    pwm.stop()
    GPIO.cleanup()
    spi.close()
```