Adding a Line-in transmitter to my car

I wanted a line-in for my car, so I bought a LineX transmitter (on right) because a friend had one and it works respectably well. The problem is that it has to be turned off when not in use because it uses batteries. It's easy to forget if stored in the glove compartment, and more easily stolen if left in plain sight.

I wanted to avoid all that by powering it from the car, locking it in my glove compartment, and having the line-in wire stick out someplace inconspicuous. One of my power ports is only on when the car is on, so devices powered from it would be automatically turned off.

Unfortunately, the car uses 12v and the transmitter needs 4.5v. A resistor might work, except the transmitter seemed to draw anywhere from 2-4 mA. Without a constant current, choosing the correct resistance would be hard. What I really needed was a 4.5v power supply, shown in the top right. It's a Velleman kit like my previous FM transmitter (top left - in an enclosure made from a printer ink box). Basically, connect a 6-40v DC or AC power source, and adjust a variable resistor until you meter 4.5v output.

You'll see from the parts that I have a couple power plugs, a matching power jack, a switch, and a project enclosure that's too small. I also added a 500 mA fuse since it would be integrated with my car (200 mA would be better, but I couldn't find anything lower).

I ended up putting everything in a larger project box and ditching the LineX enclosure. Here's a picture:

The switch cycles between batteries and external power, so I can still use it as a portable device. The two LEDs indicate power status. The red LED is from the LineX transmitter and indicates that it's transmitting. The blue 5v LED is connected to the 4.5v power converter output and indicates when the unit is running from external power. The blue LED uses more current, so only the red one is lit when using batteries. It's completely off when it's set for external power and unplugged (or the car is off):

The power converter screwed perfectly into the project box, and the LineX PCB slid perfectly into the box's side notches, except the switches to change the frequency needed an existing hole. I actually unsoldered it from the board to help size everything, which helped, but I damaged the electrical contacts in the process and had to delicately solder some bypasses. Not fun, but it works just as well. As you can see, it's a tight fit:

Here's a wiring "diagram". You'll notice the power converter is completely disabled when running from batteries. As for the switch.. basically the left and right sides are electrically isolated like two switches that switch at the same time. The center dots are either connected to the top dots or the bottom dots:

I also added an audio jack to the back because the transmitter was now in the enclosure. My brother suggested using a real antenna, which would certainly be better than squeezing a frail wire out the top for want of drilling another hole. Hopefully I can pick one up at Skycraft tomorrow on my way to meet a friend for something (so.. expect one more minor update):

If anyone's crazy enough to build another, I'll mail you my extra size M power plug, 3.5mm audio jack, several #4 screws (to mount the board to the project box), and a couple .5 amp fuses in exchange for project pictures. :P

Recommended Changes

just fyi for anyone inspired by this.. but I think if I were to make another I'd keep it simpler - no internal battery power option or switch. The LED indicating external power wouldn't be needed either- just the one I relocated that came with the transmitter which shows it is actually transmitting. I'd also probably solder in an actual resistor instead of using the variable resistor to adjust the regulator's output voltage. (The regulator converts the car's 12v power into a safer 4.5v for the transmitter, but the variable resistor for voltage adjustment is so sensitive that it's almost impossible to adjust safely - so much so that it's necessary to unsolder the connections to the transmitter first. If I were building another, I'd probably meter it and solder in an actual resistor based on that reading - no chance of bumping it to a wrong resistance/output voltage then, which means less chance of potentially frying the transmitter)