Cellphone FM Transmitter Retrofit

Describes how to add a very short range FM Stereo transmitter to a cellphone or MP3 player for use in a car.

This project is more complex than it looks. Challenges are interferences from the cellphone circuitry, the very limited space available and the circuit-to-phone interface.

The circuit uses an off-the-shelf stereo radio transmitter chip, the SN761634. It's no longer in production I think but is still readily available on Aliexpress.

Why this chip? Because it's a standalone product and can be used with a hardware determined fixed frequency without the need for serial communication. The frequency here is adjusted to 106.7MHz.

The board is a 0.4mm thick, and 9 x 11mm wide/large.

The interface to the cellphone circuitry is done via a small uC, a PIC10F204. I'm a lousy programmer, understand nothing of Android programming and the best way I found is to turn the transmitter on and off when the power button is pressed for about 6 seconds continuously (nothing that should happen under normal circumstances).

The attached Gerber files (.zip) can be used to order the PCB or you can make your own PCB, when doing that you may add a copper fill on the bottom and connect it to GND, in my case the bottom layer is empty. The PCB can be ordered from OSHPARK for a few cents, I'm still amazed about how inexpensive they are.

The assembly file (.asm) needs to be programmed into the PIC if you decide to use it. You'll need a programmer (I used a PICKIT2 clone from eBay) and software for that, I used MPlab. The circuit can be used without the PIC if you have a better idea to turn on and off the transmitter. Credit goes to David for the baseline PIC tutorials, really helpful. (http://www.gooligum.com.au/pic-tutorials)

The components could be soldered manually but I prefer using solder paste deposited on the board, then putting the components on the board and heating the board on a hotplate.

This can be soldered manually but the easiest method is to glue the board with two small drops of superglue to a hotplate. Watch the video in the last step to see the assembly process.

Then deposit some solder paste on each pad, after that place the components.

Once this is done heat the hotplate to about 480 Fahrenheit, wait for the solder paste to melt, then turn off the hotplate and let the board cool down.

After that solder wires to every pad, these are LEFT and RIGHT, ENABLE, GND, VCC and TX.

This is the first time I built the circuit and the purpose was to see how difficult it would be and what would be the range with the awfully short antenna. Since it is supposed to be used only inside a car (from phone to car radio) the lowest transmitter power setting was chosen.

I had an old LG Optimus P500 which I used for this project. I guess bigger phones may be better, there might be a little more space available. I will next try it on a Moto G.

The phone has to be pretty much completely disassembled, I recommend using something you don't need to survive.

The only spot I found where this could fit is inside the lower part of the phone which seems to be reserved for antennas.

The board is directly connected to the battery, for some reason the circuit did not work as expected with the battery voltage which is at nominal about 3.8 to 3.9V, I had to lower it by putting a 1N4148 diode in series. Due to the voltage drop of the diode the circuit now gets only about 3V and works as expected. (it transmits :-) )

The enable signal coming from the power button goes into the PIC and is compared with the voltage at the other comparator input.

When using it in another phone or MP3 player the first thing to do is to check how you want to turn on and off the transmitter. In this phone the power button signal is normally high and goes low when the button is pressed. For a positive going signal one of the commands in the attached assembly file needs to be changed. Of course you can also change the whole PIC firmware to suit your needs or you may not even need the PIC. When you look at the datasheet you'll see that the pin I used as the enable pin for the transmitter chip switches the chip from standby to 106.7MHz. This is pin 7. The IC will be in standby when the signal is low and turns on when the signal is high.

Another problem is the audio routing within the phone. The two white wires are connected to the headphone output. In this phone the headphone audio is only routed to the port when a headphone is plugged in. There is a switch inside the port that lets the phone logic know that the headphone is plugged in. That switch I had to bridge for this to work.

I know there are possibilities to route audio to the earpiece even when the headphone is plugged in, via an Android app, but that's something I didn't try out yet.

Watch the video to see it work.

Video

have fun!