GPS+Voice Interface for Wouxun KG-UV3D: Circuit Hackage

Having had my old ICOM IC-Z1A HT stop working, most likely due to the innards finally shaking loose, I replaced it with a Wouxun KG-UV3D dual-band radio. Unfortunately, the interface box I designed to connect the Byonics TinyTrak 3+ GPS modem, the helmet earbud/mic, and the external battery pack to the Z-1A doesn’t work with the Wouxun. It’s all different:

• Mechanical interface to the radio
• Battery voltage
• Power control
• Mic level
• PTT interface

I modified the interface box from my bike thusly:

Because the KG-UV3D uses the Kenwood HT interface with a single ground for mic, speaker, and PTT functions, there’s no need for galvanic isolation; all the optoisolators & the audio transformer will Go Away when I rebuild it.

The plug connections:

One distressing change: the IC-Z1A mic power was 3.5 V behind 400 Ω = 6 mA into an optoisolator LED, but the KG-UV3D puts 5 V behind 50 kΩ = 100 µA into a dead short. I think the voltage will suffice to drive a logic-gate MOSFET to switch the power through a PNP transistor, but, for the moment, I hotwired OK1 and “control” the interface power by unplugging the external battery. The radio runs from its own snap-on Li-Ion pack.

The PTT now has a separate logic wire and is no longer multiplexed as a DC current on the audio line. The hack on OK2 was the easiest way to make that happen on the existing board, but the TT3 PTT Out line can  probably drive the PTT directly.

I’m not happy with the audio levels; the KG-UV3D requires more mic gain (which change doesn’t appear in the mods) and more TT3 output. Having tediously calibrated the TT3 for the IC-Z1A, I’m not looking forward to doing that again. I still like using an analog multiplexer to switch the audio signal, though, because it doesn’t mix the machine noise with the voice transmissions.

There being no way to mount the box on the radio and no way to control the interface power if I did, I simply lashed it to the side of the pack holding the radio behind the seat. Obviously, that can’t last forever…

I think the KG-UV3D stuffs more RFI into the mic circuit, because that box is now in the only position that doesn’t result in weird voice audio dropouts. Given the precarious nature of the thing, though, I must look again after getting it in a box on the radio.

Earth to amateur radio manufacturers: seen from out here, it’d be perfectly OK to standardize some of this stuff!