Lighting LEDs with GPIOs

Description	LEDs and GPIOs Connect individual LEDs to several GPIOs in order to control them using a program. Learn how to use a loop to repeat a section of code multiple times
Materials	RaspberrySTEM CREATOR kit with Raspberry Pi LEDs RaspberrySTEM Cell
Prerequisites	None

DISCUSSION What is a GPIO? General Purpose Input/Output. A method for a computer to get or set the voltage (to high (3.3V) or low (0V, i.e. Ground) on a pin. Great for turning LEDS on/off, enabling devices (like other ICs, motors, relays, etc), or reading the state of sensors, buttons, switches, etc. GPIO pin numbers are labelled on the RaspberrySTEM Lid Connector.

ACTIVITY Hook up and light an LED Choose any GPIO pin on the RaspberrySTEM Lid Connector. The GPIO Pin numbers are written as the first number next to each pin (not all pins are GPIO pins). Connect up the circuit as shown. GPIO 14 is used in this example, but any other GPIO could be used as well.                                  Run the following program. The first two lines initialize the GPIO software. The setup() function makes the selected GPIO pin an output pin. The final line is the critical line - it turns the GPIO on (and the LED will go on as a result). import RPi.GPIO as GPIO GPIO.setmode(GPIO.BCM) pin = 14 GPIO.setup(pin, GPIO.OUT) GPIO.output(pin, True) In the output() function, modify the parameter True to False. Rerun the program - this will turn the GPIO off, which turns the LED off.

DISCUSSION Functions Functions are a named section of a program. Functions a grouped together in collections called libraries. Functions are passed a list of parameters that change how the function works. The sleep function, that is part of the time library, pauses for the amount of time given. It is called with time.sleep(seconds).

ACTIVITY Use the time.sleep() function to flash an LED. The sleep function pauses the program for the amount seconds you give it as a parameter. To use the sleep function, you'll need to add the following line to the top of your program: import time As an example, to sleep for two seconds, run: time.sleep(2) Change the previous program: Turn the LED ON, sleep for one second, turn the LED OFF, sleep for one second Repeat the above multiple times, and the LED with flash. The sleep function accepts fractional values as well - to sleep for half a second, you can use: time.sleep(0.5) Decrease the amount of time to have the LED flash faster.

DISCUSSION Simple loops: Loops are used to run a section of code for a specific number of times. All code that is part of the loop must be indented to the same level

ACTIVITY Loops allow a program to run the same section of code multiple times. The simplest loop is a “for” loop that runs a specific number of times. Open a new file. To print the word 'hello' five times, you can use: for i in range(5):         print('hello') The variable i in the above loop increases by one each time the loop is run, starting at 0. To see the value of i each loop, print it. Finally, flash the LED 10 times by adding a for loop to the previous program.

ACTIVITY Two LED Siren Add another LED/resistor to a different GPIO, as was done in the first activity. setup() your new LED, and use output() to turn it on, as was done in the first activity. Write a program to flash each LED, in an alternating fashion - when one LED is on, turn the other LED off. Flash the LED 10 times.

ACTIVITY Four LED sweep Add two more LEDs as done above for a total of four. Place them in a line. In a loop, light each LED one at a time. Back and forth: instead of lighting the LEDs just in one direction, switch direction when you get to the other side.