# LED - Let There Be Light

In this activity you will connect a Light Emitting Diode (LED) to your Raspberry Pi's GPIO (General Purpose Input Output) Pins.

Warning!

* Make sure your circuit on your breadboard is not connected to your Pi
* Double check you circuit before you connect it to your Pi

After you have created the circuit you need to write a simple Pyhon program controlling the LED state being on or off.

Your program will introduce the concepts:

* importing modules
* setting GPIO pins
* variables
* user input from screen

## Equipment Used

![](https://3057147146-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ln8HCVcRiIinj6Fl0M2%2F-Ln8HCr6xSVzXy_Qzbzq%2F-Ln8HDFVXjq6Wy6zVNnC%2FLED_2.svg?generation=1566745094327183\&alt=media) ![](https://3057147146-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ln8HCVcRiIinj6Fl0M2%2F-Ln8HCr6xSVzXy_Qzbzq%2F-Ln8HDFXCv2UfId3FvNh%2Fbreadboard.svg?generation=1566745094347004\&alt=media) Electronics:

* One LED
* Breadboard
* Male to Female Breadboard Wires

## Raspberry Pi

The model Raspberry Pi we use is the Pi 3 Model B V1.2. Older models are compatible with our programs and electronic diagrams provided you use `GPIO.Board` for the [pin numbering](https://dkoudstaal.gitbook.io/led-let-there-be-light/gpio).

### Creating the circuit

![LED circuit](https://3057147146-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ln8HCVcRiIinj6Fl0M2%2F-Ln8HCr6xSVzXy_Qzbzq%2F-Ln8HDFZgYlz1N0s7Ire%2FLedOne_bb.png?generation=1566745094205768\&alt=media)

The LED has to be connected in the right way. The electricity can only flow one way through a diode and a LED (a Light Emitting Diode). For the sake of this activity we assume the flow of electricity is from positive to negative. For a more correct explanation look at [this article](https://en.wikipedia.org/wiki/Electric_current) in WikipediA.

The anode (+) is the larger pin on the LED and the cathode (-) is the shorter pin. Also on the housing of LED is a flat part on the rim on the side of the cathode.

Pin 11 will supply the 3.3 volts to the LED when activated in your Python program.

Pin 6 is the ground pin completing the circuit when Pin 11 is turned on.

**Make sure your Pi is turned off from the power when you connect the LED circuit to your Pi!**

**Check the circuit before connecting the Pi to the power**

### A bit more about breadboards

![](https://3057147146-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Ln8HCVcRiIinj6Fl0M2%2F-Ln8HCr6xSVzXy_Qzbzq%2F-Ln8HDFbajgTu5RwraLh%2Fbreadboardexplained.svg?generation=1566745094574883\&alt=media) The vertical holes on each side of the breadboard are the Power Rails. There are four Power Rails. The holes on each Rail are connected.

The five holes on a Terminal Strip are connected.

Please note!:

* The four Power Rails are separate and NOT inter-connected.
* All Terminal Strips are separate and NOT inter-connected.
* The Terminal Strips are NOT connected across the Divider

The Divider is used for IC chips and to straddle parts such as switches, LEDs and chips.

[This site](http://computers.tutsplus.com/tutorials/how-to-use-a-breadboard-and-build-a-led-circuit--mac-54746) provides a more detailed explanation on the use of a breadboard.

### Creating the program

Every line proceeded by # is a comment and ignored by the Python interpreter.

We strongly recommend you do not cut and paste this program but type it in yourself.

```python
# led.py
# Program to turn a LED on and off.
# by Your Name

# Import the GPIO module and refer to it in our program as IO .
import RPi.GPIO as IO

# Only need the sleep function from time module.
from time import sleep

# Use the BOARD settings from the module.
IO.setmode(IO.BOARD)

# Set Pin 11 on the Pi as an output.
# This will supply 3.3 volts when turned on.
IO.setup(7,IO.OUT)

# create variables conataining a message
message_on = "Let there be light\n\n"
message_off = "Let there be dark"

# Turn Pin 11 on - the LED is supplied with 3.3 volts.
# and print a message that the light is on.
IO.output(11, True)
print(message_on)

# Wait 30 seconds. The light will stay on for 30 seconds.
sleep(30)

# Turn off Pin 11. No voltage supplied to the Pin. Light is turned off.
# And print message the light is turned off.
IO.output(7,False)
print("Let there be dark")

# Reset all the pins to the default settings
IO.cleanup()
```