Automotive Line-In Audio and USB Charging

I consider a line-in jack to be an essential feature for a car. Since I started driving, I have always had some device with audio on it that I wanted to play in the car. In my first car I put in an aftermarket stereo with line-in connections on the back, and loved it. It was kind of a hackish job with one end of an 1/8th inch audio wire connected behind the dash, and the other end sticking out of the dash, but it did what I needed it to do. However, after graduating from college, that car was nearing the end of its life. I got a new car and was once again without audio input.

I have many times gotten into the car to go somewhere only to discover that my mp3 player, cellphone, bluetooth headset, or other device is completely dead. A long drive just gets longer without good music or a podcast to listen to, and it’s nearly impossible to find a serviceable payphone around anymore. I decided I also need to have a USB charger built into my car.

An aftermarket stereo may have solved both these problems. However, the stereo in my car is perfectly good and is one of those that is integrated into the dash to the point that a replacement is either going to look very hackish or be very expensive. Besides, just buying what you need is no fun.

I very briefly considered an FM short-range broadcaster system. I’ve used these before, but they are hackish at best and tend to be pretty useless in the city where I live, as the FM spectrum is pretty saturated. My car (a Mazda 3) has support for a tape deck built into the head unit, which I hoped I could use as an auxiliary audio input. After some searching, I learned that the head unit uses a proprietary connection and protocol to interface with the external unit. Some of the connections are analog audio, but the head unit won’t use them unless it can communicate with the external device using the proprietary protocol. I was very happy to find the AuxMod project, which has already solved the problem of integrating with the Mazda radio bus. I bought one, installed it, and never looked back. It’s worked perfectly, and I’m very happy with it.

I wanted to design the USB charger system myself. Again, I’m sure there’s an aftermarket solution to do this, but the ones I found were all expensive, ugly, or both. I decided to put together a simple power converter to provide the 5 volts to a USB jack. This time I wanted to integrate the USB and audio connections into the dash somewhere. Wires dangling out from the edges of panels or hidden in the console are a hassle to dig out, are always getting in the way when they are out, look unsightly, and can’t be changed without tearing the dash apart. I wanted a nice set of connectors built into the dash to look like they were intended to be there.

After some browsing on Digikey and elsewhere, I found a great looking panel-mount USB jack. The Digikey picture makes it look like white plastic, but this is a very rugged, nickel-plated connector. It has two connectors, just in case I want to charge two devices at the same time. I also found a nice audio jack that has a similar metallic finish.

With the basics of the project fleshed out, it was time to begin designing the power supply. More on that in the next post.

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