Hi. This is a short little project I whipped up on my lunch break today.
I recently saw this hack floating around the Intertubes. The basic run-down is that an Airport Express (A1392) has it’s power supply replaced with a 12V->5V->3.3V step down sequence, so it can run off of a car’s DC power. It is then setup to pipe audio to the car’s audio system. It’s a really neat hack, I wish I had thought of this when I drove down to Florida from New England.
I decided to spin my own version of this from components I had lying around. I wasn’t keen on spending 100$ on a new Airport Express when I could whip my own up for free! I used a WAP54G, as I have many of them lying around, and it runs on 5V direct. This is nice because the RasPi also uses straight 5V, which means I’d only have to step the voltage down once.
I am not responsible for your actions if you choose to follow this tutorial. Miswiring could cause serious property damage, risks to your vehicle, and life. Seriously, be very careful. Your cigarette outlet is protected by a fast-blow fuse (on most vehicles), but that does not give you a license to do anything stupid. You’ve been warned.
Raspberry Pi with 4GB SD Card (Rasbian Preloaded)3.5mm Male to Male Cable
USB Extension Cable
Voltage Regulator (I used the LM317, but any ~14V->5V will work.)
Linksys/Cisco WAP54G V2
Garmin GPS Power Cable/Cigarette Power Adapter Without Regulator
Make sure you set Shairport to start when your Raspberry Pi boots, and test that it comes up properly. Configure your DD-WRT with a DHCP server, and disable the Internet Connection. Being that the WAP only has 1 Ethernet port, you cannot easily do other routing. However, with the WRT this is not the case. I’m not sure if you can run a DHCP server on the default Linksys firmware on the WAP, but I don’t believe you can. DD-WRT is much more powerful, I recommend it.
We start by chopping the end off of the Garmin power cable. My power cable came from a Street Pilot C550, which has a barrel jack connector. I am not covering the other type found on later Garmin models. The barrel is center positive.
Cut the Garmin adapter wire 3/4 the way to the barrel jack connector. Strip the wires on the side with the cigarette adapter. Red goes to IN+ on your LM317 board/circuit. White goes to IN-. Solder these in place, if you haven’t already. If you have an adjustable regulator, move your trimpot until your output is 5V.
Strip and solder the Red and White wires on the barrel connector side of the cable to OUT+ and OUT-. The barrel will fit into the power supply on the WAP54G! So, no other adapting necessary. If you have a WRT54G, your router uses 12V input, and this will not work at all. You will have to power your router straight from the 12V coming off the cigarette socket.
The Garmin cable has a built-in quick-blow fuse, that’s hidden under the tip of the cigarette socket adapter. If the red indicator LED on the socket adapter is mysteriously not working, check this fuse by unscrewing the cap.
Now, snip the male end of your USB extension cable off. Strip the female end’s insulation, peel back the shield, and snip the green and white wires. Solder the Black wire to the OUT- on your VREG circuit, and the Red wire to OUT+. Plug your Micro-USB cable into the Female end and test to see that your RasPi starts up. If it doesn’t, or if you see your USB device enumeration going crazy, try a different Micro-USB cable. If the problem persists, your VREG may not be supplying enough current. The WAP and the Pi together should pull close to 1 amp under full load, and a bit less when quiet.
Connect an Ethernet cable between the RasPi and the WAP, and make sure that it assigns the RasPi an IP address when the interface is brought up. Now, attach the RasPi and the VREG to the casing of the WAP somehow (I chose to mount everything on an aluminum plate… Not very efficient or elegant, but I think it looks awesomely crazy.) Tape up all of your connections to prevent shorts, and verify that it works! Name your wireless network something snazzy. I named mine ‘CarFire’. I also added a giant heatsink, since the processor on the WAP was getting pretty hot. Necessary? Absolutely not. Hilarious? Yes, very.