Lab 08: Random

Pixel says...

Pixel waves hello This time the computer chooses, and even we can't guess what comes next. We'll light one surprise pixel at a time. Ready to roll the light dice?

Program file: 08-random.py

What you'll learn

How randint() picks a random whole number
Why the top of the range is NUMBER_PIXELS-1
How three random numbers make a random color
How to turn a pixel off in memory and let the next strip.write() show it

What you'll need

Your base kit: a Pico, a breadboard, and the 30-pixel LED strip, wired as shown in the Kit User's Guide
The config.py file saved on the Pico (see Getting Code onto the Kit)
Thonny open and connected to your Pico

The program

This program lights one random pixel in a random color, waits half a second, and then picks again.

08-random.py

# Lab 08: Random
# Filename: 08-random.py
# Version: 1.0.0
#
# Light a pixel in a random color at a random location.

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

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

strip = NeoPixel(Pin(NEOPIXEL_PIN), NUMBER_PIXELS)

while True:
    location = randint(0, NUMBER_PIXELS-1)
    red = randint(0, 255)
    green = randint(0, 255)
    blue = randint(0, 255)
    strip[location] = (red, green, blue)
    strip.write()
    sleep(.5)
    strip[location] = (0, 0, 0)

Run it. One pixel at a time flashes in a surprising color at a surprising spot. You can't predict where the next one will land.

How it works

Get the random tool

This line loads randint so the program can use it.

from urandom import randint

A random number is a number picked by chance, so nobody can predict it. The randint(a, b) function gives you a random integer (a whole number) from a to b. It can return a, it can return b, and it can return anything between. So randint(1, 6) works like rolling a die.

Pick a place

This line picks which pixel will light up.

location = randint(0, NUMBER_PIXELS-1)

The pixels are numbered 0 to 29, so the last one is NUMBER_PIXELS-1. We subtract 1 because randint includes the top number. With randint(0, NUMBER_PIXELS), the program could pick 30, and there is no pixel 30.

Pick a color

These lines pick a random amount of red, green, and blue, and then color the pixel.

red = randint(0, 255)
green = randint(0, 255)
blue = randint(0, 255)
strip[location] = (red, green, blue)

Each number can be anything from 0 to 255, the full range you met in Lab 02. Three random numbers can make about 16 million different colors.

You may notice that many of the colors look pale. When the three numbers are close in size, the light looks whitish. Lab 10 uses the color wheel to fix that.

Show it, then erase it

These lines show the pixel, hold it for half a second, and then turn it off.

strip.write()
sleep(.5)
strip[location] = (0, 0, 0)

The pixel glows for half a second. Then (0, 0, 0) sets red, green, and blue all to zero. That only changes the note in the Pico's memory. No strip.write() comes right after it.

The next trip around the loop calls strip.write(). That one write shows the old pixel going dark and the new pixel lighting up at the same moment. So only one pixel glows at a time.

Sometimes the loop picks the same pixel twice in a row. Then the pixel does not go dark. It changes to its new color instead.

Try it yourself

Change sleep(.5) to sleep(.1). How does the light show feel now?
Change randint(0, NUMBER_PIXELS-1) to randint(0, 9). Which pixels flash now?

Check your understanding

Why does the program use NUMBER_PIXELS-1 and not NUMBER_PIXELS?
Which numbers can randint(0, 255) return?
How many pixels glow at the same time? Why?
The line strip[location] = (0, 0, 0) has no strip.write() after it. What makes the pixel go dark?

Lab complete!

Pixel celebrates You taught the computer to surprise you! Games, art, and music all use random numbers, and now you can too.

What's next: In Lab 09: Color Wheel, one pixel walks through every color in order.