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We'll be using CircuitPython for this project. Are you new to using CircuitPython? No worries, there is a full getting started guide here.

Adafruit suggests using the Mu editor to edit your code and have an interactive REPL in CircuitPython. You can learn about Mu and its installation in this tutorial.

Get the code from GitHub, place it in Mu and save it onto the Circuit Playground Express as code.py (not hot.py!)

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# SPDX-FileCopyrightText: 2018 Anne Barela for Adafruit Industries
#
# SPDX-License-Identifier: MIT

import time
import board
from analogio import AnalogIn
from adafruit_crickit import crickit
import neopixel

print("Heating Pad Demo")

pixels = neopixel.NeoPixel(board.NEOPIXEL, 10, auto_write=False)

def show_value(heat_val):         # Show throttle on NeoPixels on CPX
    num_pixels = int(10 * (heat_val + 0.002))
    for i in range(num_pixels):
        pixels[i] = (10*(i+1), 0, 0)
    for i in range(num_pixels, 10):
        pixels[i] = (0, 0, 0)
    pixels.show()

TMP36 = AnalogIn(board.A3)  # TMP36 connected to A3, power & ground
POT = AnalogIn(board.A7)    # potentiometer connected to A7, power & ground

heating_pad = crickit.dc_motor_2  # Set the motor object to heating pad

while True:   # Loop Forever

    voltage = TMP36.value * 3.3 / 65536.0  # Read temp sensor, get voltage
    tempC = (voltage - 0.5) * 100.0        # Calculate Celsius
    tempF = (tempC * 9.0 / 5.0) + 32.0     # Calculate Fahrenheit

    heat_value = POT.value / 65536.0   # Value (0.0 to 1.0) to drive pad

    print((tempF, heat_value))         # Display temperature and drive
    show_value(heat_value)

    heating_pad.throttle = heat_value  # set Motor throttle value to heat_value

    time.sleep(0.25)                   # Wait a bit before checking all again

# SPDX-FileCopyrightText: 2018 Anne Barela for Adafruit Industries
#
# SPDX-License-Identifier: MIT

import time
import board
from analogio import AnalogIn
from adafruit_crickit import crickit
import neopixel

print("Heating Pad Demo")

pixels = neopixel.NeoPixel(board.NEOPIXEL, 10, auto_write=False)

def show_value(heat_val):         # Show throttle on NeoPixels on CPX
    num_pixels = int(10 * (heat_val + 0.002))
    for i in range(num_pixels):
        pixels[i] = (10*(i+1), 0, 0)
    for i in range(num_pixels, 10):
        pixels[i] = (0, 0, 0)
    pixels.show()

TMP36 = AnalogIn(board.A3)  # TMP36 connected to A3, power & ground
POT = AnalogIn(board.A7)    # potentiometer connected to A7, power & ground

heating_pad = crickit.dc_motor_2  # Set the motor object to heating pad

while True:   # Loop Forever

    voltage = TMP36.value * 3.3 / 65536.0  # Read temp sensor, get voltage
    tempC = (voltage - 0.5) * 100.0        # Calculate Celsius
    tempF = (tempC * 9.0 / 5.0) + 32.0     # Calculate Fahrenheit

    heat_value = POT.value / 65536.0   # Value (0.0 to 1.0) to drive pad

    print((tempF, heat_value))         # Display temperature and drive
    show_value(heat_value)

    heating_pad.throttle = heat_value  # set Motor throttle value to heat_value

    time.sleep(0.25)                   # Wait a bit before checking all again

If you open the Mu serial terminal (press the menu bar Serial button), you'll see the temperature measured by the temperature sensor and the value of the potentiometer from 0 to 1. At 0, the heating pad is not powered, at 1.0 it is fully powered, and values between the heating pad is pulse width modulated to be at at a temperature lower than maximum.