Tone Generator with a Potentiometer

Welcome to the Tone Generator Lab

In this lab, you will turn a knob to change the pitch of a tone. Twist the potentiometer and hear the sound change in real time!

Setup

Wire up your components like this:

1. Connect the middle pin (wiper) of the potentiometer to the ADC0 pin — that is GPIO pin 26 on the Pico.
2. Connect one outer pin of the potentiometer to GND.
3. Connect the other outer pin to 3.3VREF — use the isolated GND pin next to it.
4. Connect the speaker between GND and GPIO pin 15 (lower-left corner of the Pico).

Then load and run the code below.

Tone Generator Code

import machine    # import the machine module for hardware control
import time       # import time for pausing

# Set up the potentiometer on ADC pin 26
pot = machine.ADC(26)                      # read analog voltage from pin 26

# Set up the speaker as a PWM output on pin 15
speaker_pin = machine.Pin(15)              # use GPIO pin 15
speaker_pwm = machine.PWM(speaker_pin)    # create a PWM signal on that pin

# Turn the PWM on at 50% duty cycle to produce sound
speaker_pwm.duty_u16(32768)               # 32768 is half of 65535 — 50% duty cycle

# Frequency range in Hz
MIN_FREQ = 100                             # lowest tone: 100 Hz (very low rumble)
MAX_FREQ = 2000                            # highest tone: 2000 Hz (sharp beep)

try:
    while True:
        # Read the current position of the potentiometer
        pot_value = pot.read_u16()         # returns a value from 0 to 65535

        # Map the potentiometer value to a frequency in Hz
        # When pot_value is 0, frequency = MIN_FREQ
        # When pot_value is 65535, frequency = MAX_FREQ
        frequency = MIN_FREQ + (pot_value / 65535) * (MAX_FREQ - MIN_FREQ)

        # Update the speaker frequency
        speaker_pwm.freq(int(frequency))   # freq() needs a whole number

        # Short delay to avoid updating too many times per second
        time.sleep(0.01)                   # wait 10 milliseconds

except KeyboardInterrupt:
    # Clean up when the user presses Ctrl-C to stop the program
    speaker_pwm.deinit()                   # turn off the PWM hardware
    print("Program terminated")

Code Explanation

1. pot = machine.ADC(26) — sets up pin 26 as an Analog-to-Digital Converter (ADC) input to read the potentiometer voltage.
2. speaker_pwm = machine.PWM(speaker_pin) — creates a Pulse-Width Modulation (PWM) signal on pin 15 for the speaker.
3. speaker_pwm.duty_u16(32768) — turns the sound on at 50% duty cycle. This stays on the whole time; only the frequency changes.
4. pot_value = pot.read_u16() — reads the potentiometer. The value goes from 0 (fully turned one way) to 65535 (fully turned the other way).
5. frequency = MIN_FREQ + (pot_value / 65535) * (MAX_FREQ - MIN_FREQ) — converts the potentiometer reading into a frequency between 100 Hz and 2,000 Hz.
6. speaker_pwm.freq(int(frequency)) — sends the new frequency to the PWM output. The int() call rounds it to a whole number.
7. time.sleep(0.01) — waits 10 milliseconds. Without this delay, the program would update the frequency thousands of times per second, which is not useful.
8. speaker_pwm.deinit() — turns the PWM hardware off when you stop the program. Without this, the tone would keep playing!

Key Idea

The try/except KeyboardInterrupt block is important! When you press Ctrl-C to stop the program, the except block runs deinit() to stop the PWM. Without it, the tone would keep playing after your program stops.

Monty's Tip

When you turn the potentiometer, the ADC reads the changing voltage on pin 26 and converts it to a number. Twisting the knob changes the number, which changes the frequency, which changes the pitch you hear.

Challenges

1. Change MAX_FREQ to 4000. Can you hear the full range?
2. What is the highest frequency you can still hear? (Hint: most people cannot hear above 20,000 Hz, but many already start losing high frequencies much earlier.)
3. Connect an OLED display and draw a picture of a sine wave that changes shape as the frequency changes.

Great Work!

You just built a real electronic theremin-style instrument! You control the pitch with your fingers on a knob.