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By Bob Catanzarite
swhowto.com

2.5mm Wired Headset Adapter
For the Motorola Razr Cell Phone

By Bob Catanzarite

The Motorola Razr is a very cool looking, very sexy cell phone.

I really like mine so far. One thing about it that really irritates me though is how the Razr does not have the 2.5mm jack for a wired headset that has become so common in cell phones. Sure, the Razr has Bluetooth and will work with any of the wireless Bluetooth headsets that are available. For me, these Bluetooth headsets that are available in the mid 2005 timeframe are just too expensive, too big and bulky, too awkward and too ugly. I want to be able to use the wired headset that I have been using for a couple of years. But, the Razr will not accept the 2.5mm plug that my current wired headset has. Nor does anyone seem to make an adapter that will allow these 2.5mm wired headsets to be used with the Razr.

So, I made my own. You can too but I’ll warn you, you do need to be comfortable soldering to very small parts.

The Motorola Razr has a Mini-USB connector for use with wired headsets as well as to interface via USB to a PC. Very clever and elegant on the part of the Engineers at Motorola. But bad for anyone who wants to use a wired headset with a 2.5mm plug. I plan to look deeper into the use of the mini-USB connector as an audio headset connector. If I find a better way to do this then I'll update or replace this article.

For now, the approach I took to make an adapter was to start with a headset made for the Razr by Motorola.

Check out the Motorola Website

Text Box:

Motorola SYN0896A - Headset with Send-End Key

This headset includes a Send/End key on the microphone to allow you to receive and end calls with the push of a button. This costs anywhere from $20 to $30.

The idea here is to use the circuit board inside this Motorola headset as an interface board between the mini-USB connector and a standard 2.5mm headset jack.
(Click for a larger image)

(Click for a larger image) (Click for a larger image)

There may be a way to make a simple adapter cable that would have a mini-USB connector on one end and a 2.5mm jack on the other. But until I learn more about the use of the mini-USB connector as an audio connector I chose to base my adapter on the Motorola headset. This headset has a small circuit board that has some interface circuitry along with ‘Send/End’ switch and a microphone all housed in a small plastic case. It seems that Motorola used some sort of adhesive to glue this case together, maybe ultrasonically welded it) so I had to cut the case open with a Dermal Tool which destroys the case. I didn’t see any easy way to make use of the ‘Send/End’ switch so I gave up on making use of that. What we're going to do here is to remove the built in microphone and the crappy earpiece that came with the headset and attach a standard 2.5mm headset jack to the microphone's circuit board. Put this all into a protective case.

I chose to make the part of the cable with the mini-USB connector shorter but this step is entirely optional. I wanted mine to be about 12” but you can make yours any length you choose, You can even leave the original length intact to save yourself some work. It was well worth the extra time and effort for me to make this part of the cable shorter. If you do shorten this cable you have to carefully desolder the six wires on the mini-USB cable from the circuit board. Expose the circuit board to as little heat as possible when desoldering the wires. Shorten the cable to suite your needs, strip the cable ends and carefully resolder the six wire back to the circuit board. Again, expose the circuit board to as little heat as possible. DON'T FORGET to reuse the small strain relief that was on the original part of the mini-USB cable.

Next step is to remove the microphone that came on the circuit board. You can’t keep the original mic and then add another mic as part of the wired headset. Can only have one mic. You shouldn't need to re-use the onboard mic so I didn't worry about damaging the mic. But, you NEED the circuit board so again, expose the circuit board to as little heat as possible to prevent damage.

Next step is to solder shielded leads to the circuit board that will connect to a standard 2.5mm jack. The jack I used was made to be cable mounted jack but I took it apart and discarded the plastic shell part.

(Click for a larger image) (Click for a larger image)

The shielded leads came from an old set of stereo earbuds. The black plastic case I used to enclose everything was an antistatic small parts storage case made by tbd. You can use whatever small case you have available to you. It doesn't have to be the antistatic type that I used. And it doesn't need to be shielded either. In fact you can put this together however you want. You really don't NEED a case at all. How I did it is just and example to get your creative juices flowing. I drilled a hole to mount the 2.5mm jack. The hole I drilled was tight enough that I didn’t need anything to attach the jack to the plastic case. The holes tight fit made the jack stay in place just fine for me. Just remember that my main purpose for having a case is to protect the leads that are soldered to the circuit board, to strain relief them. If you don't take care of this detail then your creation will not live long.

I made the drawing below to show where the different wires need to solder to the circuit board and how to wire in the 2.5mm jack.

(Click for a pdf image)

Enjoy!

Bob Catanzarite

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