Project 2.3

Project 2.3: Adding a NeoPixel Bar

The voltage you can already read gets something fun to control

This is for after Project 2.2’s potentiometer-reading loop is working and you’ve checked it against a multimeter. A NeoPixel stick adds a new kind of output: eight individually colored LEDs, each one addressable from code.

Wire the NeoPixel stick

A NeoPixel stick has three wires: power, ground, and one data wire that controls all eight pixels in sequence. The two ends aren’t the same, an arrow printed on the strip points from DIN (data in) to DOUT (data out), and your wire has to go into the DIN end or nothing will light up.

Data wire to GP17, physical pin 22.
Ground wire to your spare ground rail from Project 1.1, or directly to physical pin 23, a ground pin right next to the data wire, whichever is easier to reach on your breadboard.
Power wire to VSYS, physical pin 39.

Light a bar instead of just printing a number

Add the NeoPixel to the loop you already built in Project 2.2. The potentiometer-reading part doesn’t change at all, you’re just giving its result somewhere new to go.

from machine import Pin, ADC
from neopixel import NeoPixel
import time

np = NeoPixel(Pin(17), 8)   # 8 pixels, data on GP17
pot = ADC(Pin(26))          # only GP26, 27, 28 can read analog voltages

while True:
    raw = sum(pot.read_u16() for _ in range(8)) // 8
    volts = raw / 65535 * 3.3

    lit = round(volts / 3.3 * 8)   # how many of the 8 pixels to turn on

    for i in range(8):
        np[i] = (0, 20, 0) if i < lit else (0, 0, 0)   # (red, green, blue)
    np.write()   # nothing on the stick actually changes until this runs

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

Turn the knob and watch the bar grow and shrink. If you still have Thonny’s plotter open, keep volts and pixel count on it together, they share a similar scale (0 to 3.3 and 0 to 8), but leave the raw 0-to-65,535 reading out of the plot, it would flatten the other two lines to nothing.

CHALLENGE: color by voltage

CHALLENGE: COLOR BY VOLTAGE

Change the pixel loop so the lit pixels change color depending on the voltage, not just their count: green below 1.1V, yellow from 1.1V up to 2.2V, red above 2.2V. Use the volts variable you already have, not raw.

    if volts < 1.1:
        color = ...   # TODO: a green tuple
    elif volts < 2.2:
        color = ...   # TODO: a yellow tuple
    else:
        color = ...   # TODO: a red tuple

    for i in range(8):
        np[i] = color if i < lit else (0, 0, 0)

Keep the brightness values small, the same reasoning as the warning above still applies.

Going further

Once the challenge above works, here are two different ways to spread color across the whole bar instead of switching it all at once. Try both and compare.

A smooth fade, where the entire bar shifts from green to red as the voltage rises, instead of jumping between three fixed colors:

    f = volts / 3.3
    color = (int(20 * f), int(20 * (1 - f)), 0)

A fixed gradient by position, closer to how a real VU meter looks, where each pixel’s color depends on where it sits on the strip, not on the voltage directly:

    for i in range(8):
        if i < lit:
            f = i / 7
            np[i] = (int(20 * f), int(20 * (1 - f)), 0)
        else:
            np[i] = (0, 0, 0)

Both react to the same voltage, but ask yourself what each color actually depends on. One changes because the voltage changed. The other changes because a pixel’s position changed, and only the count of lit pixels depends on voltage at all.

Troubleshooting

If the strip does nothing: check that the data wire is in the DIN end, not DOUT, that the NeoPixel’s ground and the Pico’s ground actually share a connection, and that you’re powering it from VSYS, not a 3V3 pin that’s also expected to drive a weak data signal. If one pixel near the start never lights while the rest work, that pixel itself may be dead, try starting your strip from the next one over. If GP17 seems unresponsive after editing boot.py or main.py in an earlier project, a soft reset (Ctrl+D in the Shell) clears anything those files claimed on startup.

Once the bar reacts the way you expect, move on to Project 2.4, an optional part where you’ll trade the knob for a light sensor and this exact code keeps working.