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2.10 Thermistor

Introduction

A thermistor is a temperature-sensitive resistor widely used in temperature sensing and control applications. Unlike photoresistors that detect light, thermistors respond to temperature changes by varying their resistance. This makes them ideal for projects like heat alarms, thermostats, and temperature monitoring systems. In this project, we will use a thermistor to measure ambient temperature and display it in both Celsius and Fahrenheit.

What You’ll Need

Here are the components required for this project:

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

Circuit Diagram

Below are the schematic diagrams illustrating how to connect the thermistor to the Fusion HAT+:

Wiring Diagram

Build the circuit as shown in the wiring diagram below:

Ensure that:

The thermistor is correctly connected to the Fusion HAT+.
Power and ground connections are secure.
All wiring is consistent with the diagram to ensure proper functionality.

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 2.10_Thermistor.py

This Python script reads an analog signal from a thermistor via the fusion hat and calculates the corresponding temperature. When executed:

The script continuously reads the voltage.
It calculates the thermistor resistance (Rt), and determines the temperature in Kelvin, Celsius, and Fahrenheit.
The calculated temperatures are printed to the console in both Celsius and Fahrenheit, formatted to two decimal places, as Celsius: <value> C Fahrenheit: <value> F.
The process repeats every 0.2 seconds until the script is interrupted with Ctrl+C.

Code

Below is the Python code used for this project:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from fusion_hat.adc import ADC
import time
import math

thermistor = ADC('A3')

# Run the process in a try-except block
try:
   while True:
      # Read the voltage from the sensor
      Vr = thermistor.read_voltage()
      # Calculate the resistance of the thermistor
      if 3.3 - Vr < 0.1:
            print("Please check the sensor")
            continue
      else:
            Rt = 10000 * Vr / (3.3 - Vr)
            # Calculate the temperature in Kelvin
            temp = 1 / (((math.log(Rt / 10000)) / 3950) + (1 / (273.15 + 25)))
            # Convert Kelvin to Celsius
            Cel = temp - 273.15
            # Convert Celsius to Fahrenheit
            Fah = Cel * 1.8 + 32
            # Print the temperature in both Celsius and Fahrenheit
            print('Celsius: %.2f C  Fahrenheit: %.2f F' % (Cel, Fah))
            # Wait for 0.2 seconds before the next read
            time.sleep(0.2)

# Handle KeyboardInterrupt for graceful termination
except KeyboardInterrupt:
   pass

Understanding the Code

Imports:
```
from fusion_hat.adc import ADC
import time
import math
```
This script imports necessary modules: ADC from the fusion_hat library for read voltage, time for sleep functionality, and math for mathematical calculations.
Initialization:
```
thermistor = ADC('A3')
```
Initialize the ADC object on pin A3.

Data Processing:

while True:
   # Read the voltage from the sensor
   Vr = thermistor.read_voltage()
   # Calculate the resistance of the thermistor
   if 3.3 - Vr < 0.1:
      print("Please check the sensor")
      continue
   else:
      Rt = 10000 * Vr / (3.3 - Vr)
      temp = 1 / (((math.log(Rt / 10000)) / 3950) + (1 / (273.15 + 25))) # Calculate the temperature in Kelvin
      Cel = temp - 273.15 # Convert Kelvin to Celsius
      Fah = Cel * 1.8 + 32 # Convert Celsius to Fahrenheit

Reads the voltage from the thermistor.
Calculates the thermistor’s resistance.
Uses the Steinhart-Hart equation to compute temperature in Kelvin.
Converts Kelvin to Celsius and Fahrenheit.

Output:
```
print('Celsius: %.2f C  Fahrenheit: %.2f F' % (Cel, Fah))
time.sleep(0.2)
```
The calculated temperature is displayed on the screen in both Celsius and Fahrenheit. A delay is included to allow the temperature readings to stabilize and to prevent excessive CPU usage.

Troubleshooting

Temperature Values Are Incorrect or Erratic:
- Cause: Incorrect thermistor parameters or noisy input signal.
- Solution:
  - Ensure the thermistor’s resistance at 25°C (10000 in this script) and the 3950 B-value are accurate for your thermistor model.
  - Add a capacitor or software filtering to reduce signal noise.
Divide-by-Zero Error:
- Cause: The voltage Vr is too close to 0 or 3.3V, leading to division by zero in the resistance calculation.
- Solution: Ensure the ADC input voltage stays within the 0–3.3V range, and check connections for short circuits or disconnections.

Extendable Ideas

Display Temperatures on an LCD or OLED: Use an LCD or OLED screen to show the temperature values dynamically.

Data Logging: Save the temperature readings to a file for analysis:

with open("temperature_log.txt", "a") as log_file:
   log_file.write(f"Celsius: {Cel:.2f}, Fahrenheit: {Fah:.2f}\n")

Threshold-Based Alerts: Trigger alerts when the temperature exceeds a predefined threshold:
```
if Cel > 30:
   print("Warning: High temperature!")
```

LED or Buzzer Feedback: Provide visual or auditory feedback based on temperature levels:

from fusion_hat import Pin
led = Pin(27.Pin.OUT)
if Cel > 30:
      led.on()
else:
      led.off()

Conclusion

This experiment demonstrates how to use a thermistor with an Fusion HAT+ to measure ambient temperature accurately. By understanding the principles of analog-to-digital conversion and the Steinhart-Hart equation, you can build advanced temperature monitoring and control systems for various applications.