Project 2.2

Project 2.2: Reading a Potentiometer, and What a Voltage Divider Is

What a knob is actually telling the Pico

This project asks a question the last one skipped over: when you read a potentiometer, what is the Pico actually measuring? Knowing the real answer is what makes the next few projects (a NeoPixel bar, then a light sensor, then a joystick) all work with almost no new ideas, just the same one applied again.

Wire the potentiometer

Wire it the same way Project 2.1 wired the servo’s power, except this time the middle pin (the wiper) goes to GP26, and the two outer pins go across your power and ground rails so it reads across the full range.

Never power the potentiometer from VBUS or VSYS. Those are 5V, and the Pico’s ADC can only read up to 3.3V, so a reading above that just clips at the maximum and stops responding to the knob. It’s an easy mistake to repeat with the joystick later, which is wired the exact same way.

A 1k and a 10k potentiometer read identically here. The ADC measures a ratio of voltages, not a specific resistance, so the exact value doesn’t matter as long as both outer legs are connected.

Read the knob into a number

from machine import Pin, ADC
import time

pot = ADC(Pin(26))   # only GP26, 27, 28 can read analog voltages

while True:
    # Read the pot 8 times and average them. A single reading jitters a
    # little; averaging smooths that out.
    raw = sum(pot.read_u16() for _ in range(8)) // 8

    # read_u16() gives 0 to 65535, where 65535 means the full 3.3V.
    volts = raw / 65535 * 3.3

    print("Volts:", round(volts, 2))
    time.sleep_ms(100)

If sum(pot.read_u16() for _ in range(8)) looks unfamiliar, here’s the same averaging written as an explicit loop instead:

total = 0
for _ in range(8):
    total += pot.read_u16()
raw = total // 8

Run it, then open Thonny’s View > Plotter. Turn the knob and watch the live plot respond. The text before the number in print() becomes that line’s label on the plot.

Prove it with a multimeter

The code claims a specific voltage. Check it.

With everything still running, measure the actual voltage from the wiper (GP26’s wire) to ground with a multimeter. Compare it to what Thonny is printing.
Unplug the Pico. Measure the resistance from the wiper to each outer leg of the potentiometer. Call the one going to ground R_bottom and the full end-to-end resistance R_total.
Calculate 3.3 * R_bottom / R_total by hand and compare it to what you measured in step 1. This is the actual voltage-divider formula, the same ratio your code is approximating with read_u16().

Your 3V3 rail might measure closer to 3.25V than exactly 3.3. That’s real: the code assumes 3.3 because that’s the nominal, advertised voltage, not because it’s guaranteed to be exact. Worth noticing that your program has been running on an assumption the whole time.

Once turning the knob reliably changes the voltage you’re printing, and you can explain why, move on to Project 2.3, where that same number finally has somewhere fun to go.