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2.1.7 Potentiometer(MCP3008)

Note

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

Introduction

The ADC function is used to convert analog signals into digital values. In this experiment, we use the MCP3008 ADC chip to perform this conversion. A potentiometer is used to generate a variable voltage, which changes the physical quantity. The MCP3008 then converts this analog voltage into a digital value that can be read and processed by the Raspberry Pi.

Required Components

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Raphael Kit	337	Raphael Kit

You can also buy them separately from the links below.

COMPONENT INTRODUCTION	PURCHASE LINK
GPIO Extension Board	BUY
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
LED	BUY
Potentiometer	BUY
MCP3008	-

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.

Note

Please place the chip by referring to the corresponding position depicted in the picture. Note that the grooves on the chip should be on the left when it is placed.

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: Open the code file

cd ~/raphael-kit/python

Step 4: Run.

sudo python3 2.1.7-2_Potentiometer.py

After the code runs, rotate the knob on the potentiometer, the intensity of LED will change accordingly.

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 raphael-kit/python. After modifying the code, you can run it directly to see the effect.

#!/usr/bin/env python3

import spidev
import time
import RPi.GPIO as GPIO

# GPIO pin for PWM output
PWM_PIN = 22

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

# Initialize PWM on GPIO22 at 1000Hz
pwm = GPIO.PWM(PWM_PIN, 1000)
pwm.start(0)  # Start with 0% duty cycle

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

def read_adc(channel):
    """
    Read analog value from MCP3008
    :param channel: ADC channel (0-7)
    :return: 10-bit integer (0-1023)
    """
    if channel < 0 or channel > 7:
        return -1
    adc = spi.xfer2([1, (8 + channel) << 4, 0])
    value = ((adc[1] & 3) << 8) | adc[2]
    return value

def MAP(x, in_min, in_max, out_min, out_max):
    """
    Map a value from one range to another
    """
    return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min

try:
    while True:
        # Read analog value from CH0
        res = read_adc(0)
        print('res = %d' % res)

        # Convert to 0–100% duty cycle
        duty_cycle = MAP(res, 0, 1023, 0, 100)

        # Update PWM duty cycle
        pwm.ChangeDutyCycle(duty_cycle)

        time.sleep(0.2)

except KeyboardInterrupt:
    pass

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

Code Explanation

1. RPi.GPIO is used to generate PWM signals to control an LED. spidev is used for SPI communication with the MCP3008. time is used to add delays in the loop.

   #!/usr/bin/env python3
   
   import spidev
   import time
   import RPi.GPIO as GPIO
   

2. Configure GPIO pin 22 for PWM output using RPi.GPIO. Set up SPI communication with the MCP3008 (Bus 0, CE0) at 1 MHz.

   PWM_PIN = 22
   
   GPIO.setmode(GPIO.BCM)
   GPIO.setup(PWM_PIN, GPIO.OUT)
   
   pwm = GPIO.PWM(PWM_PIN, 1000)  # 1kHz frequency
   pwm.start(0)  # Start with 0% duty cycle
   
   spi = spidev.SpiDev()
   spi.open(0, 0)
   spi.max_speed_hz = 1000000
   

3. This function reads analog data from the MCP3008 on the specified channel (0–7) using the SPI protocol. The result is a 10-bit integer ranging from 0 to 1023.

   def read_adc(channel):
       if channel < 0 or channel > 7:
           return -1
       adc = spi.xfer2([1, (8 + channel) << 4, 0])
       value = ((adc[1] & 3) << 8) | adc[2]
       return value
   

4. This function maps a value from one numerical range to another. It’s used to scale ADC values to PWM duty cycle percentages.

   def MAP(x, in_min, in_max, out_min, out_max):
       return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min
   

5. In the main loop, the program continuously reads analog data from channel 0 of the MCP3008, maps the value to a PWM range (0–100), and sets the LED brightness accordingly. The loop updates every 0.2 seconds. If interrupted (e.g., Ctrl+C), the program stops the PWM signal and cleans up the GPIO configuration.

   try:
       while True:
           res = read_adc(0)
           print('res = %d' % res)
   
           duty_cycle = MAP(res, 0, 1023, 0, 100)
           pwm.ChangeDutyCycle(duty_cycle)
   
           time.sleep(0.2)
   
   except KeyboardInterrupt:
       pass
   
   finally:
       pwm.stop()
       GPIO.cleanup()
       spi.close()