Converting ADC Values to Temperature

Learn how to accurately convert ADC readings from the Pico's temperature sensor into temperature values in degrees Celsius.

Introduction

The Raspberry Pi Pico’s ADC provides raw digital values that correspond to the temperature of the RP2040 chip. To make these readings meaningful, we need to convert them into temperature values in degrees Celsius. In this lesson, you’ll learn how to use a mathematical formula for this conversion.

Step 1: Understanding the Conversion Formula

The built-in temperature sensor measures the voltage output of the chip’s temperature sensing circuit. This voltage is then converted to an ADC value, which can be transformed into a temperature in degrees Celsius using the formula:

temperature = 27 - (voltage - 0.706) / 0.001721

Explanation:

• 27 is the nominal room temperature in degrees Celsius.
• 0.706 is the reference voltage at room temperature.
• 0.001721 is the voltage change per degree Celsius.

Step 2: Reading and Converting ADC Values

Update your script to include the conversion formula:

import machine
import time

# Initialize the ADC for the temperature sensor
sensor_adc = machine.ADC(4)

while True:
    # Read the raw ADC value
    raw_value = sensor_adc.read_u16()
    
    # Convert the ADC value to voltage
    voltage = raw_value * 3.3 / 65535

    # Convert voltage to temperature
    temperature = 27 - (voltage - 0.706) / 0.001721

    # Print the temperature
    print(f"Temperature: {temperature:.2f}°C")
    time.sleep(1)

Step 3: Running the Script

1. Save the script as convert_adc_to_temp.py.
2. Run the script in Thonny.
3. Observe the temperature readings in the console, now displayed in degrees Celsius.

Step 4: Verifying the Readings

To ensure the readings are accurate:

• Compare the readings with a thermometer in the same environment.
• Note that the Pico’s sensor measures the temperature of the chip, which may be slightly higher than the ambient temperature due to heat generated by the chip.

What You’ve Learned

• How to use a formula to convert ADC values to temperature.
• How to write a script that continuously displays temperature readings in degrees Celsius.

Example Output

Temperature: 26.84°C
Temperature: 26.85°C
Temperature: 26.87°C

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