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2.8 Potentiometer

Introduction

The ADC (Analog-to-Digital Converter) function is essential for converting analog signals to digital ones. In this experiment, we use the Fusion HAT+ to achieve this conversion, utilizing a potentiometer. The potentiometer adjusts the voltage, a physical quantity, which the ADC then digitizes. This experiment also demonstrates how the potentiometer can control the brightness of an LED.

What You’ll Need

Here are the components required for this project:

COMPONENT INTRODUCTION	PURCHASE LINK
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
LED	BUY
Potentiometer	BUY
Fusion HAT+	-
Raspberry Pi	-

Circuit Diagram

Below are the schematic diagrams for the project:

Wiring Diagram

Build the circuit as shown in the image below:

Running the Example

All example code used in this tutorial is available in the ai-lab-kit directory. Follow these steps to run the example:

cd ~/ai-lab-kit/python/
sudo python3 2.8_Potentiometer.py

This Python script uses an Fusion HAT+ to read analog input and control the brightness of a PWM LED. When executed:

The Fusion HAT+ continuously reads an analog signal, converting it to a digital value between 0 and 4095.
This value is mapped to a range between 0 and 100, representing the brightness percentage of the LED.
The brightness of the PWM LED is dynamically adjusted based on the mapped value.
The raw ADC value and voltage is printed to the console in real time.
The program runs indefinitely until interrupted with Ctrl+C, at which point the LED turns off.

Code

Below is the code for the experiment:

#!/usr/bin/env python3

from fusion_hat.adc import ADC
from fusion_hat.pwm import PWM
import time

# Set up the potentiometer
pot = ADC(0)

# Initialize a PWM LED
led = PWM(0)

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

try:
   while True:
      # Get the current reading from the ADC port
      result = pot.read()
      voltage = pot.read_voltage()
      print('result = %d voltage = %.2f' %(result,voltage))

      # Map the ADC value to a range suitable for setting LED brightness
      value = MAP(result, 0, 4095, 0, 100)

      # Set the LED brightness
      led.pulse_width_percent(value)

      # Wait for 1 seconds before reading again
      time.sleep(0.2)

# Graceful exit when 'Ctrl+C' is pressed
except KeyboardInterrupt:
   led.pulse_width_percent(0)  # Turn off the LED

Understanding the Code

Imports:
```
from fusion_hat.adc import ADC
from fusion_hat.pwm import PWM
import time
```
The script uses fusion_hat for PWM LED control and analog-to-digital conversion, and time for implementing delays.

Initialization:

# Set up the potentiometer
pot = ADC(0)

# Initialize a PWM LED
led = PWM(0)

The script initializes a PWM LED on pin P0 and an ADC on pin A0.

MAP Function:

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

The MAP function converts values from one range to another, essential for mapping ADC readings to LED brightness.

Main Loop:

try:
   while True:
      # Get the current reading from the ADC port
      result = pot.read()
      voltage = pot.read_voltage()
      print('result = %d voltage = %.2f' %(result,voltage))

      # Map the ADC value to a range suitable for setting LED brightness
      value = MAP(result, 0, 4095, 0, 100)

      # Set the LED brightness
      led.pulse_width_percent(value)

      # Wait for 1 seconds before reading again
      time.sleep(0.2)

# Graceful exit when 'Ctrl+C' is pressed
except KeyboardInterrupt:
   led.pulse_width_percent(0)  # Turn off the LED

Continuously read ADC values.
Map ADC readings (0-4095) to brightness levels (0-100).
Adjust LED brightness and wait for 0.2 seconds before repeating.

Troubleshooting

LED Does Not Respond
- Cause: Incorrect wiring or GPIO pin configuration.
- Solution: Ensure the LED is connected to PWM 0 with an appropriate resistor.
ADC Values Always Zero
- Cause: Incorrect wiring of the sensor.
- Solution: Verify the sensor connections. Ensure the input sensor is functioning.
Mapping Issues
- Cause: Incorrect MAP() function parameters.
- Solution: Ensure the input range (in_min, in_max) matches the output range (0–4095) and the output range (out_min, out_max) is suitable for LED brightness control (0–100).

Extendable Ideas

Threshold-Based Behavior: Add logic to turn the LED on or off based on specific ADC value thresholds:
```
if result > 2048:
   led.on()
else:
   led.off()
```

Conclusion

This experiment demonstrates how to utilize the Fusion HAT+ and a potentiometer to control the brightness of an LED. By understanding analog-to-digital conversion and PWM control, you can expand this knowledge to build more complex interactive systems.