Snap Crackle Pi

For the speaker I used pretty much the exact circuit used by BrunoIP in his instructable here. The only change is that I used the 5V supply from the Raspberry PI. R1 is 4.7KOhm, R2 is 1M. Q1 can be a 2N3904 (NPN) and Q2 can be 2N3906 (PNP). C1 is a 10uF electrolytic capacitor. Oh and I threw in the obligatory power LED with a 1K resistor between +5 and the anode.

As you can see from the photo of my project I just threw it together on a mini breadboard and put it in a high tech enclosure (tupperware with a hole cut in the lid into which I hot glued the speaker).

Step 1: Get Raspian image.

Step 2: Burn image to an SD card.

Step 3: Transfer the fireplace WAV file to the raspberry pi.

• Set up your Pi with a keyboard and display, and download the file from the internet using a browser running on your Pi.
• Or, set up your Pi the completely headless way, log in to it, and then use wget to fetch the file. For example:

wget -O fireplace.wav http://soundbible.com/grab.php?id=1543&type=wav

• Or (for example, if the audio file you want requires log in to download--like this one from freesound.org which is the one I am actually using), use scp to transfer the sound file to your Pi.

Step 4: Play sound at boot up, by creating /etc/init.d/fireplace.sh file:

#!/bin/sh
while [ true ]; do aplay /home/pi/fireplace.wav; done;

Now make this file executable:

sudo chmod 755 /etc/init.d/fireplace.sh

Now add this script to all the defaut runlevels:

sudo update-rc.d fireplace.sh defaults

Reboot and you should be all set!

If you don't like the volume, you can log in to your Pi and then run the command line mixer:

alsa-mixer

The save your settings to a file using the alsactl ("also control") program:

mkdir ~/.config
 alsactl --file ~/.config/asound.state store

Then you will want to load these settings before you loop your sound file at boot by editing your fireplace.sh script as follows:

#!/bin/sh 
alsactl --file /home/pi/.config/asound.state restore 
while [ true ]; do aplay /home/pi/fireplace.wav; done;

Next up: an amplified speaker to hear the sounds.

I have the system up on running, complete with low density polyethylene speaker enclosure (aka "Tupperware"). In the next few days I will post photos and schematic, as well as a run down on how to get your raspberry pi up and running without using an attached keyboard or monitor. This is all old-hat for most of you, but for the uninitiated may be useful for future reference.

I spent quite a bit of time hunting around for the simplest possible amplifier that would run off of 5 volts. There are lots of examples of suitable Class A amplifiers out there, but I wanted a Class B amplifier to reduce the quiescent current draw on the Raspi. Most Class B amplifiers seemed to be designed for higher voltages, but I found a nice example of the simplest conceivable low voltage class B amplifier here:

http://www.instructables.com/id/Rechargable-pocket-sized-amplifier/

(The author there further cites "Lupin, introduccion a la electronica", but I am not sure at this moment what exactly that is.)

Class B amplifiers can distort low amplitude components of the audio signal due to "crossover distortion"--i.e, the signal is neith positive enough to push nor negative enough to pull, remembering that transistors do not conduct at all when the base voltage is < 0.7 volts. But in this case, any distortion just adds to the ambience. A class AB amplifier would be better (it uses a bias voltage to ensure that at any given moment on or the other transistor is in the conduction zone), but would nearly double my part count!

I found a very nice tutorial on amplifier basics here:

http://www.electronics-tutorials.ws/amplifier/amp_6.html