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Wikijunior:Raspberry Pi/Raspberry Pi LEGO® Christmas Tree LED Lights

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Tutorial by Andrew Oakley - Public Domain
Nov 2017 www.cotswoldjam.org

LEGO® is a trademark of the LEGO Group of companies which does not sponsor, authorise nor endorse this tutorial nor Cotswold Raspberry Jam.

Components

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Light emitting diodes (LEDs)

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A Light Emitting Diode (LED) has a short leg and a long leg. If you feel around the rim, you'll also find a flat edge. The short leg and flat edge always connect to negative (ground).

  • ×1 red LED
  • ×1 blue LED
  • ×1 yellow LED

Resistors

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The resistor can be connected any way around. Lower value (lower Ohm) resistors will allow more current through and will make the LED brighter; higher values will let less current through and make it dimmer. We're using 270 Ohm resistors, but anything between 220–470 Ohms will work.

  • ×3 270 Ohm (red violet brown) resistors.

Jumper wires

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You will need jumper wires to connect the LEDs to the resistors and then to the Raspberry Pi.

  • ×6 10cm female-to-female jumper wires
  • ×3 20cm female-to-female jumper wires

LEGO® bricks

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Finally, you will need two LEGO® bricks to make the Christmas tree.

Connecting the LEDs and resistors

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Take two short (10cm) female-to-female wires and place a resistor between them.

Do this three times so you have three long wires with a resistor in the middle.

Next, attach the short resistor wires that you just made, to the negative (short) leg of each LED.

Finally, attach the long (20cm) female-female wires to the positive (long) leg of each LED.

Bend the legs of the LEDs so that you can fit them around the LEGO® fir tree.

Connecting to the Raspberry Pi

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Connect the wires with resistors to any of the GND (Ground, Negative) pins. These are shown in black on the diagram to the right.

Next, connect the positive wires – the long wires without resistors – to the following pins. GPIO 14 (pin 8) is on the fourth pin from the top-left of the Raspberry Pi when the Ethernet port is to the right:

  • Red LED – GPIO 14 (pin 8)
  • Blue LED – GPIO 15 (pin 10)
  • Yellow LED – GPIO 18 (pin 12)

Don't worry about the colours of the wires. Concentrate on the colours of the LEDs. GPIO stands for General Purpose Input Output. These pins have many uses.

The program

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On your Raspberry Pi desktop, click the Raspberry menu (top right), then select Programming and click on Python 3 (IDLE).

Click File menu, Open. Double-click the python folder (you may need to scroll right to find it). Double-click the lego-xmas folder. Select lego-xmas1.py and click Open.

A new window will open. In the new window, select Run menu and click Run Module. The red LED should light up briefly, and then turn off. Let's have a look at this program. 

from gpiozero import LED
from time import sleep
red=LED(14)
red.on()
sleep(0.5)

What is this program doing?

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from gpiozero import LED
from time import sleep

The computer doesn't know how to do everything itself; it has to learn. These two lines teach the computer about LEDs and waiting (sleep). The places where these lessons are kept, are called Libraries. We import the LED command from the gpiozero library, and the sleep command from the time library. 

red=LED(14)
red.on()

We have to tell the computer which GPIO pin the red LED is attached to, and then we tell it to turn the LED on. 

sleep(0.5)

This tells the computer to wait (sleep) for half a second.

When the program ends, it turns off the LEDs automatically. This auto-off feature is a part of the gpiozero library.

Lighting all the LEDs in sequence

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Close the program by clicking on the lego-xmas1.py window, then select File menu, Close.

From the main window, open the lego-xmas2.py program, then run it with Run menu, Run Module.

This program lights all the LEDs one by one, waiting half a second between each.

Stop the program by holding down the CTRL key and pressing C

What is this program doing?

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Notice how we define the pin numbers for the red, blue and yellow LEDs, and we switch them on and off in order. 

from gpiozero import LED
from time import sleep

red=LED(14)
blue=LED(15)
yellow=LED(18)

while True:
  blue.off()
  yellow.off()
  red.on()
  sleep(0.5)

  red.off()
  blue.on()
  sleep(0.5)

  blue.off()
  yellow.on()
  sleep(0.5)

The while command repeats a section. We say while True to mean "forever".

Notice how the commands beneath the while command are indented by a couple of spaces from the left. This is called a block. The indentation lets the computer know which commands it should repeat as part of the while loop.

Also notice that while True: ends with a colon :.

The colon lets the computer know that it should expect a block next.

Try changing the sequence. Swap some of the on() and off() commands.

Change the sleep length to longer or shorter. Try adding extra on(), off() and sleep commands for different LEDs.

Make your own amazing Yuletide light sequence!

Advanced extras

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The program lego-xmas3.py shows you how to use the pulse command. Notice how lego-xmas3.py uses the PWMLED library rather than just the simple LED library. 

from gpiozero import PWMLED

The pulse command is more difficult to use, because you have to time the pulses of your different coloured LEDs so that they match up with your sleep time! 

 red.pulse(0.3,0.3,4,True)
 blue.pulse(0.4,0.4,3,True)
 yellow.pulse(0.6,0.6,2,True)
 sleep(2.4)

The program contains some comments which explain how the pulse command works.

Teacher Preparation

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Downloads available from http://www.cotswoldjam.org/downloads/2017-11

To add the lego-xmas*.py program files to Raspbian, go to the terminal: 

cd
mkdir -p python
cd python
curl "http://www.cotswoldjam.org/downloads/2017-11/lego-xmas.zip" -O
unzip lego-xmas.zip
exit

Note that -O is the capital letter O, not zero. More tutorials available from http://www.cotswoldjam.org/tutorials

Files

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Cjam-lego-xmas.pdf

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The original PDF for this tutorial is on Wikicommmons: Cjam-lego-xmas.pdf

lego-xmas1.py

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from gpiozero import LED
from time import sleep

red=LED(14)
red.on()
sleep(0.5)

lego-xmas2.py

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from gpiozero import LED
from time import sleep

red=LED(14)
blue=LED(15)
yellow=LED(18)

while True:
  blue.off()
  yellow.off()
  red.on()
  sleep(0.5)

  red.off()
  blue.on()
  sleep(0.5)

  blue.off()
  yellow.on()
  sleep(0.5)

lego-xmas3.py

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from gpiozero import PWMLED
from time import sleep

red=PWMLED(14)
blue=PWMLED(15)
yellow=PWMLED(18)

while True:
  red.pulse(0.3,0.3,4,True)
  blue.pulse(0.4,0.4,3,True)
  yellow.pulse(0.6,0.6,2,True)
  sleep(2.4)

# PWMLEDs have a pulse command. The parameters are:
# 1 Fade in time, in seconds
# 2 Fade out time, in seconds
# 3 Number of times to pulse
# 4 Background - True to continue to the next command without waiting
#                False to wait until finished
#
# For example:
# red.pulse(0.3,0.3,4,True)
# * Fade in for 0.3 seconds
# * Fade out for 0.3 seconds
# * Repeat 4 times
# * Continue with the next command without waiting
# 4x0.3=1.2 seconds to fade in
# 4x0.3=1.2 seconds to fade out
# 1.2+1.2=2.4 seconds total
#
# Find out more about PWMLED commands
# http://gpiozero.readthedocs.io/en/stable/api_output.html#pwmled
#
# Learn about PWM Pulse Width Modulation
# https://learn.sparkfun.com/tutorials/pulse-width-modulation
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