Friday, November 20, 2009

Installing Bluetooth audio in my car with a Motorola s705

I am nearly done with my project to create a seamless way of playing music that is stored on my computer in my car. I bought a Motorola s705, which is a Bluetooth audio receiver with headphone jack. It is capable of operating as a standard phone headset and also capable of receiving high-quality audio. First, let me review the s705 itself:

I easily paired the s705 with my HTC G1, and was able to play music through the s705 with no trouble. The sound quality was extremely good -- significantly better than the G1's own headphone adapter. It appears the high-pass filtering that the G1 uses on its own audio stream are thankfully not also used on the bluetooth high-quality audio stream. I could not hear any compression artifacts, and the sound quality remained top-notch as I walked up to about 15 feet away from the phone. I did not test the s705's battery life. I accidentally turned on the FM radio a few times, and I wish that feature could be disabled. The only negative thing is that the s705 and G1 sometimes do not connect to each other after being separated, then brought back together. Sometimes, they will only establish a phone audio connection and not a high-quality (A2DP) connection. Turning the phone off and on (or going into airplane mode and back) always fixes the problem. Turning the s705 off and on sometimes fixes the problem.

So after I decided the s705 would probably do the job, I took it apart (of course). I needed a way to remotely control the s705 power (to have it turn on everytime I started the car, and off when I shut car off), and also a way to allow it to charge its own battery while not running the car battery down.

In order to turn the s705 on or off, the power switch must be held down for three seconds. One side of the power switch would go high (about 4 volts) when the switch was pressed. Thus, I could control the device's power state by sending it a 4 volt pulse that lasted three seconds. My first plan was to use an Atmel AVR microcontroller and use a feedback circuit so that the AVR would know whether the unit was on or off. I had difficulty finding a reliable logic signal on the s705 circuit board that could be used as an on/off indicator. I was also having difficulty with the Atmel IDE that night, so I scrapped it, and decided to use two basic timer circuits (a 555 and 558 chip). The circuits send a pulse when the car turns on, and another pulse when the car turns off. It's possible for the device to become out of sync, but I don't think it will happen. The circuit uses a MOSFET whose gate is driven by a capacitor-resistor network and charged by the "on" signal from the car stereo. This way, the device will draw essentially zero current after the gate discharges and the MOSFET turns off. I've configured the capacitor value so that the circuit stays powered for 15 seconds after the car turns off.

I also wanted to sneak the audio wires out the back of the unit instead of using the headphone jack on top. I figured this would look a lot nicer when I mounted the unit on my car's dashboard.

I used the s705 shirt clip to attach it to an unused switch plate in my 1992 Honda Civic. It fits right next to the rear defroster switch.

The row of header pins have the following signals:
1. gnd
2. +12V always on from the battery
3. +5V signal from car stereo when the aux input is active
4. audio signal gnd
5. audio left
6. audio right

The audio connections go to my car stereo "aux input" which I described in another post:

The shirt clip was ABS plastic as was the switch dummy plate. I used some Weld-On #16 glue which will very securely bond ABS plastic.

It's a pretty clean installation. I've just finished it, and I'll let everyone know how it works in day-to-day use. So far, my tests in the driveway have been really great -- just step into the car and turn on the stereo, hit play on the s705, and the music starts. I don't even have to take the phone out of my pocket.


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  2. Hi Ben:

    I`ve just got a motorola s705 but it is screwed because of using a defective audio cable so i`ve got the connector`s tip inside the s705 and I just watched this and wonder if you can make a tutorial on how to disassemble it without breaking anything (plastic tabs, parts, etc.) and put it back together so I can troubleshoot mine and go ahead with this project.

    Thanks in advance

    -Jorge Meza