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Flowing Light

Control a 10-LED bar graph to create a flowing light animation — LEDs light up one at a time from left to right, then right to left, repeating in a loop.

New Concepts

  • Lists

Lists

Python lists hold a sequence of values, for example l = [1, 2, 3] is a list holding the values 1, 2, and 3. Values in a list can be access by index, so l[0] accesses the first item in the list, 1 using the example from above.

See Python documentation for more info on lists.

Component List

Component List

Component Knowledge

LED Bar Graph

A LED bar graph integrates 10 individual LEDs into a single compact component. Each LED has its own pair of pins at the bottom — one anode and one cathode — just like a standalone LED.

LED bar graph component

Circuit

Wiring Diagram

Disconnect all power before building the circuit. Reconnect once verified.

Wiring Diagram

If the LED bar does not light up, try rotating it 180°. The label orientation is not standardized across all units.

Schematic Diagram

Schematic Diagram

Each LED anode connects to its GPIO pin. Each cathode connects to GND through a 220Ω resistor.

Code

File: `01_first_examples/code/FlowingLight.py

import time
from machine import Pin

pins = [21, 47, 48, 38, 39, 40, 41, 42, 2, 1]

def showled():
    length = len(pins)
    # Left to right
    for i in range(0, length):
        led = Pin(pins[i], Pin.OUT)
        led.value(1)
        time.sleep_ms(100)
        led.value(0)
    # Right to left
    for i in range(0, length):
        led = Pin(pins[(length - i - 1)], Pin.OUT)
        led.value(1)
        time.sleep_ms(100)
        led.value(0)

while True:
    showled()

How to Run

Online

  1. Open Thonny → 01_first_examples/code/.
  2. Double-click FlowingLight.py.
  3. Click Run current script — LEDs sweep left to right, then right to left.

Code Explanation

Pin list

pins = [21, 47, 48, 38, 39, 40, 41, 42, 2, 1]

Stores the 10 GPIO pin numbers in order from left to right. Using a list makes it easy to iterate over all LEDs.

Left-to-right sweep

for i in range(0, length):
    led = Pin(pins[i], Pin.OUT)
    led.value(1)
    time.sleep_ms(100)
    led.value(0)

Turns each LED on for 100ms then off, moving forward through the list.

Right-to-left sweep

for i in range(0, length):
    led = Pin(pins[(length - i - 1)], Pin.OUT)
    led.value(1)
    time.sleep_ms(100)
    led.value(0)

Iterates forward through i but accesses the list in reverse using length - i - 1.

Key Concepts

  • Lists: store multiple GPIO pin numbers for easy iteration
  • len(list): returns the number of items in a list
  • range(start, end): generates a sequence of integers for the for loop
  • Reverse indexing: pins[length - i - 1] accesses list items in reverse order without reversing the list itself

Reference

for i in range(start, end, step=1)

Iterates from start up to (but not including) end, incrementing by step each time.

range(0, 5)      # → 0, 1, 2, 3, 4
range(0, 10, 2)  # → 0, 2, 4, 6, 8

Back to Module

Adapted from Python_Tutorial.pdf Project 3.1