Getting LEDs to light up with Raspberry Pi's GPIO with Python

In order to get into some hardware related projects with the RASPI, I figured it would be a good idea to figure out how to do some very basic stuff with the GPIO.

The first project I think anyone does is make the LEDs light up. There are plenty of tutorials out there with bits of information scattered about, but I'll try to use my experiences to supply all the information (and references to information) here.

First of all, it is a good idea to know what each pin does on the Raspberry Pi GPIO and get your Pi all ready and set up to work with Python.


Here is a great tutorial via adafruit that gives you a quick and dirty overview the GPIO, steps to take and libraries to install in order to program it via Python.

Once you have that all set up, in order to make an LED light up programmatically, you need to have it wired properly between a resistor that is connected to one of the numbered GPIO pins (4, 8, 7, 17, 22, 23, 24 or 25),  and the ground pin (GND).

I wired my LEDs using the adafruit Pi Cobbler and a breadboard, but you could attach wires directly to the GPIO if so inclined.

Here is the example I followed for connecting LEDs to the Pi's GPIO.

And here is mine wired up to three LEDs:

In this example I have my LEDs wired up to GPIO ports:

• GREEN_LED = 24
• RED_LED = 23
• BLUE_LED = 18

The resistors are the ones that came with my kit and are rated at 560 ohm 5%, and resistors rated between 330 ohm and 1000 ohm are fine.

Here is a handy resistor decoder I use now and then.

In the above picture, the black wire is contacted between the GND pin and the negative blue column on the bread board.

Here is a diagram (note the totally fake cobbler)

For the red LED, the resistor is connected to GPIO pin 23 and an unused row on the breadboard (row 18..chosen arbitrarily). The positive end of the LED (long wire) is also in row 18 and the negative end (short wire) is connected the the blue negative column where the ground wire is connected.

I did the same for the other two LEDs but with their respective GPIO ports. Green was further away from the GPIO pin, so I used the yellow and green jumper wires to make a similar circuit.

Once everything was connected, I wrote a simple Python program to cycle through the LEDs. Somehow it worked!

Here is the code, and then some 'splainin':

#!/usr/bin/env python

import RPi.GPIO as GPIO, time, os

SLEEP_TIME = 1

GPIO.setmode(GPIO.BCM)
GREEN_LED = 24
RED_LED = 23
BLUE_LED = 18

GPIO.setup(GREEN_LED, GPIO.OUT)
GPIO.setup(RED_LED, GPIO.OUT)
GPIO.setup(BLUE_LED, GPIO.OUT)

try:

    while True:
        print "Red Light"
        GPIO.output(GREEN_LED, False)
        GPIO.output(RED_LED, True)
        GPIO.output(BLUE_LED, False)
        time.sleep(SLEEP_TIME)  
        
        print "Blue Light"
        GPIO.output(RED_LED, False)
        GPIO.output(BLUE_LED, True)
        GPIO.output(GREEN_LED, False)
        time.sleep(SLEEP_TIME)          
        
        print "Green Light"
        GPIO.output(BLUE_LED, False)
        GPIO.output(GREEN_LED, True)
        GPIO.output(RED_LED, False)
        time.sleep(SLEEP_TIME)
        
except KeyboardInterrupt:
      GPIO.cleanup()
        

Break it on down:

GREEN_LED = 24 

24 is the GPIO pin. I don't like to hard code values and plus it makes code more readable/maintainable when you do things this way.


GPIO.setup(GREEN_LED, GPIO.OUT)

Initialize the GPIO pin used for the green LED for output


GPIO.output(GREEN_LED, True)

Turn the LED on


GPIO.output(GREEN_LED, False)

Turn the LED off


GPIO.cleanup()

Uninitialize all the GPIO pins

And there you have it!

Here is a little Python code that pings an IP address. Once an IP address pings back, an LED is turned on. In this example, when one of three iPhones is detected on my home network, an associated LED will light up. The light will turn back off if the IP address becomes unreachable.

#!/usr/bin/env python

# This program will detect specific IP address on the network and 
# Enable an LED if it is detected 

import RPi.GPIO as GPIO, time, os
from subprocess import call


SLEEP_TIME = 1

host1 = "192.168.1.128" # my iPhone
host2 = "192.168.1.108" # wife's iphone
host3 = "192.168.1.186" # an iPad

GPIO.setmode(GPIO.BCM)
GREEN_LED = 24
RED_LED = 23
BLUE_LED = 18

GPIO.setup(GREEN_LED, GPIO.OUT)
GPIO.setup(RED_LED, GPIO.OUT)
GPIO.setup(BLUE_LED, GPIO.OUT)

def detect_IP(ip_add, color_led):
 ping_command = "ping -c 1 %s > /dev/null 2>&1" % ip_add
 
 if call(ping_command, shell=True) > 0:
  GPIO.output(color_led, False)
 else:
  GPIO.output(color_led, True)
 
try:

    while True:
        detect_IP(host1, GREEN_LED)
        detect_IP(host2, RED_LED)
        detect_IP(host3, BLUE_LED)
        time.sleep(SLEEP_TIME)  
        
except KeyboardInterrupt:
      GPIO.cleanup()