I have a stereo on my car, but it doesn't have bluetooth, so I though well, why not add it?
Step 1: Dongle
I ordered a couple of bluetooth dongles from eBay for $14 delivered from china, with AD2P support which was the required feature.
Taking it apart.
Step 2: Circuit Analysis Overview
I took a series of macro photos and combined then in a single macro picture.
Step 3: Removing the Connector
the dongle is pretty damn small, but the 3.5mm connector takes most of the height of the dongle.
Since we're not using it, removing it is very easy.
Just heat the surface mount pads with a soldering iron while using some metallic tool to pry the connector at the same time you heat the pad. Repeat for each pad until the connector pops out.
Step 4: Preparing the Car Stereo
Taking the front panel apart, and see what's inside, and planning the hack.
I wanted to make everything just on the front panel of the car stereo, I wouldn't want to take the stereo out of the car to do this.
The panel already have Aux in and usb port so audio and power is available just on the front panel. which is handy.
Step 5: Hack
After planning the best placement I started to put it toghether.
Step 6: Dealing With Interference and EMI
This is a very complicated topic, I had some issues, with those dongles, because they are not very well designed.
But after some experimenting, I managed to solve it.
You can see here, my first attempt to add a couple of ferrite beads savaged from a old diskette drive.
Step 7: Bypass Cap
Fiddling around I found that if I could bypass the circuit with a very large capacitor, It was going to do. But I really had no room for a large enough cap.
My first attempt, was to remove the aux connector and use the same room to fit a 440uF cap, but in the end the front panel couldn't fit, because even when the cap was the same height as the connector, the connector goes out of the case. I had to remove the cap and start to think in another solution.
So i put the connector back in place.
Step 8: Low-pass Filter
I had a small hap between a mooting hole and a plastic support, to fit a small cap 220uF.
This time I tried to put the capacitor in a decoupling configuration with a series resistor acting as a low-pass filter.
Also added a extra turn in the ferrite beads.
And this time it worked!
Cranked the volume all the way up, and a barely audible noise.
Step 9: Antenna
The dongle already have a built in antenna, and it can be enough when it is on the original case made 100% of plastic. But being tucked behind a mult-layer copper board and also sandwiched in front of the metallic body of the main stereo, the antenna will not be very efficient for sure.
To calculate the correct length of the antenna, need to know the wavelength of the desired frequency, witch in this case is 2.4GHz for bluetooth. λ = v/f λ is the wavelength, v is the speed, and f is frequency. v is C = speed of light. For 2.4Ghz we get λ = 93.75 mm
I built a 1/4 (λ) wavelength monopole antenna is very simple with just a coax cable.
Step 10: Building the Antenna
I had this WiFi antennas savaged from a old laptop, left over from my other instructables. The tiny coax cable is basically the only one that could be useful in this project. Although the antennas are nice and could be used to bluetooth because WiFi use the frequency as bluetooth, I don't had enough room to fit then, instead I will use just the coax cable to built my own antenna.
I drilled a hole near the antenna trace, and on the other side scratched the solder mask on the ground plane to create a ground pad.
and soldered the coax cable to the ground through the hole and the antenna trace.
I placed the antenna on the place more far from the metal and that would be more exposed to the front.
I tested the range and I could go 10 meters outside the car with my iPhone 5S without cutting the streaming, with the doors closed, which is a big deal because the car is a RF cage.
Step 11: Putting It Back Together
Just snap everything in place, the car stereo looks original from outside, and the hack is easily reversible.