A Cell Phone Headset Adapter for Amateur Radio

By

Geoff Haines, N1GY

            A few months ago, I wrote about an adapter I cobbled up to make use of inexpensive PC headsets for use with ham radios. The article was published by QST and I got some feedback indicating that some hams were building similar devices. One respondent in particular asked if it would work with a cell phone headset. At the time, I replied that I was unsure, but it sounded like an idea that was worth pursuing, someday.

            As luck would have it, the other day I found a cell phone type headset in a local discount store at a price ($3.00) that was too good to pass up. The purchase made, the project quickly followed.

            After getting no help at all from the cell phone manufacturer nor from the supplier of the headset, a little careful disassembly of the headset determined that the mic element was connected to the tip of the 2.5mm Stereo plug. The earphone was wired to the ring and ground for both was the shaft of the connector. This was verified with a second, different headset.

            Using the same basic circuit as the PC headset adaptor, the only change required was to tie both the mic ground and the chassis ground together. This left only three wires to go to the jack for the headset. One comes from the junction of the resistor and capacitor for the electret element and goes to the tip connection on the jack. The second comes from the audio feed from the connector that goes to the external speaker jack on the radio. The third goes from the point where the chassis ground and the mic ground tie together to the ground connection on the jack. Looking at the circuit diagram will make it all clear.

            The PTT and UP/DN switches are wired as in the PC adaptor and the whole thing terminates in a CAT-5 cable of the appropriate length along with a cable for the external speaker connection. The one change here from the original article is to use the two unused wires in the CAT-5 cable (pins 3 and 8 are not used in normal operation) for feeding the audio from the external speaker or “phones” jack on the radio to the headset. This requires only minor surgery to the CAT-5 cable about 3 to 6 inches back from the RJ-45 connector, slitting it enough to locate and expose the wires for those two pins. These wires are cut and the wires past the cut point away from the RJ-45 are spliced onto the two conductors for the connection to the external speaker connector. Suitably insulated and supported with heat shrink, this splice causes no weakness in the cable and reduces the cabling to the adapter to one CAT-5 cable with a short tail to go to the phones jack.

            Although this adapter was designed for the Icom IC-706 series of radios, there is no reason it could not be built for other radios that use an electret mic element as well. An 8-contact connector is an 8 contact connector whether it is an RJ-45 or the more traditional round 8-pin type. The pin-outs may be different for different connectors or brands of radio, but the circuit still works once those pin designations are accounted for.

            Now that the unit has been constructed, the big question is “How does it work?” The answer is “amazingly well”. Reports of audio quality have been remarkable with some operators saying that it equals or betters my Heil Traveler headset. Comments like “full range audio” and “sounds fantastic” made my ego swell, however anecdotal they may be. I do not have the facilities to do full-on audio quality testing or analysis, so I will be satisfied that hams who have heard me use the stock Icom mic, the Traveler headset, the PC headset and now the cell phone headset, say this last one is the full equal of any of the others.

In addition to the low cost, the much smaller size of the typical cell phone head set makes it more comfortable to wear for long nets. It is much lighter, and since the mic element and its little wind cap are beside my cheek instead of in front of my face, it doesn’t interfere with the occasional sip of icy beverage during the time spent in front of the radio. (Non-alcoholic, of course!)

In summary, I should note that I had success with the same values for the resistor and capacitor used for the PC headset electret element. This, I think, was pure luck. You may have to adjust the value for the capacitor to achieve best audio response from the mic. The resistor value is usually found by looking at the specs for the radio in question. Use the same value as the stock mic impedance. The value for the second resistor is also specific to the radio. In some radios it may not be needed at all. Just review the stock wiring of the hand mic that came with the radio to see if it is required and if so, what value it should be. The enclosure for the switches and components is up to you. I have used Radio Shack project boxes, candy mint tins, film canisters and the like. Use whatever fits the budget and works. Update: I have wired (and use) cell phone headset adapters for the Alinco DR-150T as well as the Icom IC-706MkIIG and the Yaesu FT-7800R, FT-8800R series of radios. All reports that I have received as to their performance are outstanding.

UPDATE: I have decided to offer the simplified version of this device (without the channel up/down buttons) for sale on a as requested basis. If you would like me to build you one similar to the one in the picture at the top of this page, please send me an e-mail (to n1gy@arrl.net) specifying the brand and model of radio, the type of mic connector used, and the position of the external speaker jack (front or back). The unit will be supplied with a cellular headset (over the ear type) with a foam windscreen for the mic and a foam pad for the earphone. The unit will be ready to plug in and use. My projected price for the complete unit using a RJ-45 mic plug will be $45.95 plus shipping. If you want an 8-pin round mic plug or the unit is for use with a Yaesu transceiver that has the MH-48 as its stock mic there will be an extra charge for the different connector and/or added electronic components.

 I will make one last comment about the use of CAT-5 cable and RJ-45 connectors for this and other projects. There is some controversy over the resistance of CAT-5 cable to RFI. This appears to be caused by too much RF in the immediate vicinity of the radio room.  All I can tell you is that I have been using very inexpensive CAT-5 or CAT-3 cables and connectors for years with my two Icom 706MkIIG’s, my Yaesu FT-7800 and my Alinco DR-150T and have yet to have any problems at all.  Keep building, this is what makes amateur radio special.