Lab 5: Fill Colors

Pixel says...

One pixel is cute. 256 pixels is a party! But a big party needs big power, so we'll do some math first. Let's light this up!

Program file: 05-fill-colors.py

What you'll learn

• How a for loop with range visits every pixel
• How to fill the whole matrix with one color
• How much power lit pixels use
• Why the kit has a LEVEL setting
• How to check that your power supply holds up

What you'll need

• Your kit, with the matrix wired as shown in the Kit Guide
• config.py saved on the Pico
• Thonny open and connected to your Pico

The program

This program fills the whole matrix with red, then green, then blue, then white. Each color stays for one and a half seconds.

05-fill-colors.py

# Test 05: Fill Colors
# Filename: 05-fill-colors.py
# Version: 1.0.0
#
# Fills all 256 pixels with red, green, blue and white in turn.
# The color numbers stay small (LEVEL in config.py) because every pixel
# is lit at once.
# Not yet tested on hardware.

from machine import Pin
from neopixel import NeoPixel
from utime import sleep
import config

print("Test 05: Fill Colors (version 1.0.0)")

# hardware settings from config.py
NEOPIXEL_PIN = config.NEOPIXEL_PIN
NUMBER_PIXELS = config.NUMBER_PIXELS
LEVEL = config.LEVEL

strip = NeoPixel(Pin(NEOPIXEL_PIN), NUMBER_PIXELS)

colors = [
    ("red", (LEVEL, 0, 0)),
    ("green", (0, LEVEL, 0)),
    ("blue", (0, 0, LEVEL)),
    ("white", (LEVEL, LEVEL, LEVEL)),
]

while True:
    for name, color in colors:
        print("Fill:", name)
        for i in range(NUMBER_PIXELS):
            strip[i] = color
        strip.write()
        sleep(1.5)

Run it. The whole matrix changes color every one and a half seconds. The colors are dim on purpose. If your Pico disconnects or the lights flicker when white appears, stop the program and read the power section below.

Test 05: Fill Colors (version 1.0.0)
Fill: red
Fill: green
Fill: blue
Fill: white

This picture was drawn by a computer simulator, so your real matrix may look a little different.

How it works

Read the setting

LEVEL = config.LEVEL

config.LEVEL is a number in your config.py file. In this kit it is 8. All four colors use LEVEL as their biggest number, like (LEVEL, 0, 0) for red. If you change LEVEL in config.py, every color follows.

Fill every pixel

for i in range(NUMBER_PIXELS):
    strip[i] = color
strip.write()

range(NUMBER_PIXELS) counts from 0 up to 255. That is 256 numbers, because the last number is not included. The loop sets every pixel to the same color. Then one strip.write() sends all the colors at once.

The power math

Every pixel has three tiny lights inside. Each light uses a little electricity. A milliamp (mA) is a small amount of electric current. One pixel at full white, (255, 255, 255), uses about 60 mA. That works out to about 0.078 mA for each color number.

A USB (Universal Serial Bus) port gives about 500 mA. Let's check the white fill:

Step	Math	Answer
Color numbers in one white pixel	8 + 8 + 8	24
Color numbers in 256 pixels	256 × 24	6,144
Current	6,144 × 0.078	about 480 mA

That is just under the 500 mA that USB supplies. White is the biggest fill in this program, so LEVEL = 8 is the right size. Now see what full power would need:

Step	Math	Answer
Color numbers in one pixel at full white	255 + 255 + 255	765
Color numbers in 256 pixels	256 × 765	195,840
Current	195,840 × 0.078	more than 15,000 mA, or 15 amps

Watch out!

Never fill all 256 pixels with big numbers like 255. A USB port cannot supply 15 amps. The Pico may restart, or the wires and the matrix could get hot.

Try it yourself

1. Work out on paper how many mA the red fill uses at LEVEL = 8. Use the table above as a guide. The answer is about 160 mA.
2. Work out on paper what the white fill would need if LEVEL = 16. Do not run it. Compare your answer with 500 mA.
3. Change the order of the colors in the list. Which color do you want to see first?

Check your understanding

1. What does range(NUMBER_PIXELS) count from and to?
2. Why does the program use LEVEL and not 255?
3. Which fill uses the most power: red, green, blue, or white? Why?
4. What would happen if all 256 pixels used (255, 255, 255)?

Lab complete!

You lit all 256 pixels and kept the power safe. That's math protecting your kit!

What's next: In Lab 6: Walk the Pixels, one pixel visits every spot to show the numbering.