My little stupid Raspberry Pi project: 1 LED and 1 Switch

I recently bought a Raspberry Pi. My goal is to build a living room mp3 player. As I’m a software guy the programming part isn’t a problem, but the mp3 player has to be able to respond to some buttons having a semantic like: “Hey, next song please!”. So I have to do some simple hardware. Sadly, my knowledge about electronics has faded. This is the journey of electronics ignorant in hardware land.

As the a first step, I put two examples from the raspbian user guide together. Pressing a switch toggles a LED, enabling and disabling it. Right now I’ve no idea how to calculate the pull-up-resistor for the switch and the resistor for the LED. I was happy to see that it simply worked.



Parts: Breadboard, 10k Ohm, 150 Ohm, Switch, 1 Led, Jumper Wires

What I learnt so far

There are four and five band resistor color codings. One half of the resistors I use are four band coded and the other half five band coded. Man, this is like software world with downward compatibility. Can’t they just use five bands?

The world outside of a computer has x,y,z axis. There is a difference between nice, short and too f***ing short cables. Because they had only male jumper wires at my store, things are a bit improvised anyway. I had to fit an end connector pin on the male wire. The guy at the shop said I should solder it on. Yeah right, the first thing I’ll do is soldering; adding some shrink tubing is also a good idea. The x,y,z problem also applies to switches. They actually have to fit into the Breadboard.

Software-wise the project is very simple. I use the raspberry pi GPIO python library. I read the state of the switch from port 12 and enable the LED on the first and disable the LED on the second state change.

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BOARD)
GPIO.setup(11, GPIO.OUT)
GPIO.setup(12, GPIO.IN)
state = False

def wait():
 time.sleep(0.1)
while True:
 wait() #don't burn the CPU
 if not GPIO.input(12):
    print state
    state = not state
    GPIO.output(11, state)
    while not GPIO.input(12):
            wait()
            continue

Not very surprisingly it’s a good idea to not poll the input port constantly. Otherwise the python process would use 100% of the available CPU time. I don’t know yet how to replace the polling with GPIO interrupts.


I think I won the first round of the hardware game:
a) It works.
b) My Raspberry is still alive.
c) It’s quite clear I have to learn and practice a lot.