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.

How to restore the subversion history of a file

Every now and then the history of a file in a subversion repository is lost. I’ll describe how you can restore the history.

If you do a svn copy the file history of the new location shares the history of the old location. This is way subversion supports svn move and branches. Restoring a file’s history is only a matter of copying the right file. Given the file $A_FILE to restore, the new file location $B_FILE and the last revision $A_REV of  $A_FILE (before $A_FILE was removed) the operations to restored the history of $A_FILE are:

svn delete $B_FILE
svn cp $URL/$A_FILE@$A_REV $B_FILE
svn cat $URL/$B_FILE > $B_FILE

After you restored $A_FILE’s history all changes directly to $B_FILE are lost. Only the content of the file $B_FILE survives.

Here’s a commented script demonstrates the problem and show how to fix it:

#!/bin/bash -ex

#do everything in a working directory that's removed after the script ran
mkdir -p work
WORK=$(readlink -f work)
cd "$WORK"
trap 'rm -rf "$WORK"' EXIT SIGINT SIGTERM

#setup a repository
REPOSITORY=$(readlink -f repo)
svnadmin create "$REPOSITORY"
URL=file://$REPOSITORY

#checkout the local workspace
svn co $URL workspace
cd workspace

A_FILE=a.txt
B_FILE=b.txt

#create the file to restore
echo $A_FILE content > $A_FILE
svn add $A_FILE
svn commit -m"initial version of $A_FILE"
svn update
A_REV=$(svnversion)

#history of the file is lost
svn delete $A_FILE
svn commit -m"deleted $A_FILE"

#the new file
echo $B_FILE content > $B_FILE
svn add $B_FILE
svn commit -m"initial version of $B_FILE"
svn update
B_REV=$(svnversion)

#restore the file history and keep the file content
svn delete $B_FILE
svn cp $URL/$A_FILE@$A_REV $B_FILE
svn cat $URL/$B_FILE > $B_FILE
svn status
svn commit -m"history from $A_FILE in new path $B_FILE"

#check the result
svn update && svn info
svn log --diff $URL/$B_FILE
svn cat $URL/$B_FILE
svn diff $URL/$B_FILE $URL/$B_FILE@$B_REV