15 September 2006

Bluetooth headset modification

Ok, this is not really 100% home made, because it's just a modded "behind the ear" BT headset, but it was still a cool and rewarding project.

Why did I do it?
Well, the original BT headset was kind of crappy. People were complaining that they can't hear me properly, and the audio volume was not the best either. Plus, wearing it behind the ear is not very comfortable, at least not for me.
Another reason is that I like to hack things, just for the fun of it.
I want to mention that this is a prototype, so it's not looking as good as it could. I use it only at home, so the aesthetic aspect was totally unimportant for me. All I cared about was for it to be comfortable and reliable.

1. I guess it is obvious that I am not responsible if you decide to give it a try and damage something/someone.
2. While working with Lithium Ion batteries, ALWAYS wear eye protection. I usually don't follow warnings, but this is a warning I ALWAYS follow. Those batteries can be very mean, and if you shortcut them they can explode and burn stuff. While burning your hands is unpleasant, it is at least repairable. Your eyes are harder to repair.

Thing you will need:
1. A BlueTooth headset that you don't care about.
2. A pair of headphones with a microphone. The pair I used was broken, and they didn't work (I suspect the problem was a broken wire near the connector).
3. Optional: A bigger Lithium Ion battery.
4. Optional: A solar panel that can output ~6V and a blocking diode.
5. Optional but very useful: A multimeter.
7. Soldering iron, solder.
8. A pliers, screwdriver, anything you can use to open the damn thing. More on this later.

The BT headset I used is a Logitech HS02-V07. It is pretty cheap, usually under 30 USD.
Now as you might guess, each headset is different, and might have different requirements for a microphone. So there is a risk that the microphone your BT headset uses is totally incompatible with the one in your headphones, in which case you will get bad noise, bad sound, etc. So again, this is a risk you have to take. But my model worked fine with the microphone from my headphones.

The first step, which is also the most dangerous, is to open the BT headset. They are pretty small, and usually they don't have screws. Everything is glued together.
What I did was get a pair of pliers and press on the region near the microphone at an angle that caused the glue bound to break. Then I inserted a flat screwdriver and pushed the case aside. This operation requires a great deal of attention, because everything is so small and a wrong move can damage the circuit board.
Here is how the opened headset looks like:

Next we will focus on the headphones. What we want to do is to cut the wire leading to them about 15 CM from the headphones end. Most of the headphones with a microphone have 5 wires, 3 for the headphones, 2 for the mic. My specific pair had the mic wires together, in an isolated casing (see the picture for details)

What you want to do is group the wires together, the ones for the speakers, and the ones for the microphone. The speakers have 3 cables, one for the left, one for the right, and one is the ground for both of them (common). So a simple way to connect them is by ignoring the ground wire, and connect the BT speaker output to the left and right headphone speakers. This will have the effect that they will be connected in series with your BT headset, which doubles their impedance. Don't worry about that, a mismatched impedance in this context is more that adequate for voice. But of course, you can connect them in parallel if you want, or just use a resistor in parallel with the speakers to reduce the impedance.
Connecting the microphone is easy as well, just remove the original microphone and connect the wires to the headphone's mic wires.

Now that we have the wires connected, it's a good idea to do a test. Start the BT headset, connect it to whatever device you are using (PC, PDA, cellphone, etc.) and see if you get audio in the headphones, and then try to call someone to evaluate if the mic is working properly.
Alternatively, if you are using a computer, you can start Sound Recorder (or the equivalent for your OS), set the input and output to use the BT connection, and record something then listen to it. If you hear a lot of noise in the background, don't worry, Skype for example removes all the noise.

The following steps are optional, and not necessary for the project, but it's cool nonetheless.
The original battery was very small. This is not surprising, as the size and weight of the headset is very limited. No one wants to put a brick on their ear.
I don't know how many mA the original battery has, but I would guestimate that not more than 300 and not less than 200. So while this battery can run your headset for quite a while, why not use a bigger battery, especially if you have one that you don't really use?
It happened that I had a bigger Lithium Ion battery from an MP3 player that was broken. I don't know how many mA it can store, but I guess it's about twice the capacity of the original battery.
I also happened to have a few unused 6V solar panels, and since they are pretty small and lightweight, I thought it would be cool to use one on my new headset, just for the hell of it. I mean, who else has a solar powered BT headphone with a microphone? :)

So basically just glue the Lithium Ion battery on the back of the solar panel, as shown. A multimeter is very useful, you want to wire the + of the solar panel to the + of the battery, and the - to the -. A blocking diode should be placed between the battery and the solar panel to allow the electricity to come in the battery, but not to go out in the solar panel. A multimeter is useful to determine that.
See the image on how mine looks like:

What we need to do next is cut the wires from the original battery, and connect them to the new battery. Usually red is +, black is -, but just to be on the safe side, use the multimeter to determine the polarity.
You might need an 'extension cable' to go to the new battery, depending how far the circuit board is from the new battery.
One side note, if you are using a solar panel and a blocking diode, be sure to connect the wires from the circuit board battery input directly on the +/- of the new battery, before the diode. Otherwise the diode will prevent the current flow to your BT.

We are almost done now. Do a final test to make sure everything works, then glue the battery to the headphones top. Note: if the bluetooth won't start, connect the power supply to the power port, then try again.
One other thing, it seems that at least in my case, removing the battery cleared the memory so now my computer won't ask me for the pairing key. I am not sure if other headsets will do the same thing.

Now all you have to do is to use some adhesive tape and glue the extra wires to the frame of the headphones, so they won't hang allover.

My device is a prototype and looks like one, therefore it can be considered ugly. That doesn't bother me, as I use it only at home, for Skype. But if you like cute looking devices, you have various options, such as building a small box for the project, cut the wires at the exact length so they won't hang out, etc.

The sound quality is very good for VOIP, but if you think about listening to music with it, forget about it and get some stereo BT headphones. This was not meant for anything but voice.
The sound is louder, and since the microphone is closer to your mouth, the other party can hear you better as well.
The whole project took about two hours, and it was pretty fun. So I recommend it to others. And remember, even if you fail, you still get some experience :)


Anonymous Anonymous said...

I read the post and is all interesting, but I'm an EL player too and my attention go here:
2. While working with Lithium Ion batteries, ALWAYS wear eye protection. I usually don't follow warnings, but this is a warning I ALWAYS follow.

This Warning is interesting and I think can be usefull to solve a EL's problem: the unsold items.
In these days we can se a run to buy ingredients for high lvl items because some friendly mixers will make it for free.

So, why don't obstruct this way?
A player can own, for example, a hydrogenium bar only if he has knowledge, nexus, and alchemy lvl to handle it. In this way we cut off 2nd characters or players don't interested to work but only to have high lvl items quickly and a low cost (friendly mixers) and, on the other hand, we can have producers more happy to sell the final product in competion with other producers.
In real life we don't go in the Logitech's factory with ingredients, but we buy the final product (good or bad... :) )

Best Regards,

16/9/06 11:43  

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