Ham Radio

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Car Bluetooth Audio

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The Problem

The car I own (Ford Focus C-MAX, 2004 gate release) has a Sony CD/MP3 player which I installed. The MP3 CD Player is very good, and, with MP3s it is easy to get around 150 songs onto a single 700MB file. However, I regularly use my mobile phone as a source of music when on trains, walking about, etc, and so I regularly update the collection. I wanted to get my phone music to play via the car's speakers, as my phone has 32GB of space, and will hold a much larger music collection.

When I installed the radio, I noticed two RCA connectors on the radio back - clearly inputs or outputs. Playing with the radio's menu narrowed this down to them being inputs, as there is an option for aux input (and no mention about sub-woofer outputs). My Dad's Ford Mondeo has a 3.5mm jack socket inside the glove box, where it is possible to connect a mobile phone, iPod, or similar. My C-MAX didn't have this option and Ford do not offer any wiring looms or kits to help.

I set about making a cable with two RCA connectors on one end for left and right connections to the Sony radio. The other end had a 3.5mm socket, which was fixed into the top of the glove box in a similar fashion to my Dad's Mondeo has. This works perfectly, and I have been using it for the last 3 years. Recently, I decided to re-visit this idea further to a conversation with my Dad over a cafe lunch one Saturday (yes, we're both Electronic Engineers, and so this kind of topic is common). I was explaining that the problem is, when the phone is playing music, the volume is controlled (as expected) by the amplifier in the Sony CD head unit, but the track playing is changed on the phone - very difficult (and illegal) to do when driving.

The solution suggested was to search for a wiring kit to allow the audio to be sent via Bluetooth, which provides a path for the audio and a way of controlling the mobile phone. This is discussed in subsequent sections below. I could not find an official way of getting this to work and interfacing nicely with the car's radio front panel - I wanted to be able to use the Next, Back, Pause, Play, buttons on the Sony radio to control the phone. I needed to interface the car radio to the Bluetooth receiver, as well as patching the audio from the Bluetooth receiver back into the aux inputs for the car. I had an idea...

An Idea

Two Bluetooth profiles, A2DP (Advanced Audio Distribution Profile) and AVRCP (Audio/Video Remote Control Profile) are usually used for this purpose. They are common in Bluetooth headphones, hands free kits, etc. The first profile, A2DP, provides a basic method to send high quality audio over Bluetooth - our music. The second, AVRCP, provides a way of feeding back control information - the controls.

If I could buy a Bluetooth module that performed the tasks required, it could be built into a small project box and interfaced to the car radio. This could be hidden out of view, and with the mobile phone paired to the module, the phone music could be played and controlled via the car radio. This is what I wanted.

The Bits

After extensive hunting, I found several devices - Amazon has many. Many receivers are designed for home audio equipment, which enabled the user to play music via a Hi-Fi system. However, these required mains electricity, and did not offer the ability to control the mobile phone. I also looked at buying a PCB module as a subsystem, as I have a Bluetooth serial port PCB module. These were quite expensive, and were vaigue about their supporting of AVRCP. I finally settled on a cheap Bluetooth headset thing from eBay (Auction Title: Mini Clip-On A2DP Stereo Bluetooth Headset Headphones, from seller Zimplex-UK). It was cheap and nasty looking, but was fundamentally cheap, so it was okay! I have purchased it, and am waiting for it to arrive from China. The image below is of the device I purchased - it will be taken apart for the components inside and repackaged up to fit in the car.

Ebay Auction Image

The Plan

As touched on above, the headphone unit will be taken apart. Wires will be attached to each of the buttons on the Bluetooth receiver, so that they can be interfaced to the car radio. The audio will also be patched through to the car radio's aux input. An LM7805 (fixed voltage regulator) will be used to drop the car's 12VDC to the 5VDC required for the Bluetooth receiver to work. This will all be built into a small project box, with just the interface wires and power coming from the box itself. This will then be attached inside the glove box of the car.

With some luck, and a little skill, it will all work! Bingo!!

This page is due for an update when the headphone thing arrives from China (11-23 days shipping estimate).

So Far

The item arrived some months back and I have not really played with it. I recently placed it in the car as is, and started using it. It works very well, but I plan to modify it to be more suitable for use in the car. The plan has been revised somewhat; I will revise the plan when I get some more time.