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ID-51: VOX for APRS

I hate Icom’s not HAM friendly policy but if you want to try D-Star you had no big choice before the TH-D74 from Kenwood was finally released. So you could get an ID-51 and found that there is no APRS on it. Don’t let to be fooled: D-PRS is not the APRS at all. However you’ve heard that for casual APRS operations you don’t need a real TNC but you may simply connect your any VHF radio to a phone (or to PC) with an almost no-cost homebrew cable. Yes, it is true. TNC is software emulated on a phone and works well. You just need the VOX in your radio to start transmitting. And then you find that ID-51 has VOX but the VOX does not work. To activate VOX you must purchase an adaptor from Icom for about $30. But wait, for about $30 you may buy a chinese dualbander with working VOX!

The solution is very simple: just add one resistor!

Circuit description.
Resistors R1, R3 and R4 used to reduce audio level from headphone outputs to microphone inputs. Exact resistance may vary depending on your phone and radio volume control so these resistors are not absolutely needed. A pair of R3 and R4 used to isolate left and right audio outs and also aren’t exactly necessary. However, better design requires potentiometers to set the attenuation correctly.
Capacitors used to break DC from microphone power supplies.
Resistor R5 is needed for the phone to recognize a headset. Recommended resistance is 2k2 but no great tolerance is required for most phones.
Resistor R2 is needed for the radio to recognize a $30 adaptor cable and allow the VOX. Any amount from 51k to 62k should work fine, 56k is recommended.
Transformers are not absolutely necessary but it is a good idea to get the galvanic isolation especially if you’re going to connect the radio to a PC and not to a phone only. I’ve used common “audio 1:1” transformers.

On the radio side are two TRS connectors. Thinner 1/10” is used as audio in. Tip is for signal, sleeve as a ground. Ring not connected; there is 3V power out from the radio to external devices so never connect ring to mic in or to ground. Thicker 1/8” is used as audio out and for VOX. Tip is audio out, sleeve is a ground. Ring is used to enable VOX.

On the phone side is one TRRS connector. Tip and first ring are left and right audio out respectively. Second ring is ground. Sleeve is a mic in. Note that some phones are using classic pinout where second ring is mic in and sleeve is ground: in that case just swap mic and ground. There is no restriction to make two or even more cables in parallel for different devices however I’m pretty sure the better idea is a TRRS female jack and some custom cables for your equipment.

That’s all.

7667615 Last modified: 2016-10-31 19:51:48, 3211 bytes

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