FT817ND VOX Interface

A month ago I built a rough copy of the Wolphi VOX interface to go between my Yaesu FT-817 transceiver and my Android Tablet - its shown on my now largely defunct Facebook blog. It worked OK with the Tablet but was somewhat "hit & miss" on my Chinese copy Samsung phone. The Wolphi interface uses the DC bias available on the Tablet audio port to power a single stage audio amplifier, it's normally used to supply an electret microphone in a headset.

It would appear this DC level is not standard across all Android devices and on my phone it was a dismal 1.4V, far too small to provide a decent rail to power the audio amp. This resulted in my interface failing to key the FT-817 in several of the digital-modes I tested, even with the phone volume set to maximum. 

Needless to say I was rather disappointed, but to be fair if I'd used a Darlington pair transistor rather than an FET to control the PTT line, as in  the original design, it would have worked. The FET requires twice as many volts to put it into conduction - so there's nothing wrong with the Wolphi design. I'd decided to use an FET because it's is less likely to be biased on by stray RF fields which are often present in the shack  and portable installation where RF grounds are less than perfect. If  the interface were to be effected then the transceiver PTT line would hang in transmit mode, because the PN junction in the control transistor rectifies the RF and biases itself permanently on.

I finally settled on a circuit designed WA8LMF which uses a step up transformer rather than an audio amplifier to generate the high level of audio needed to drive the VOX circuit and in order to make the rectifier circuit more efficient I used a Schottky diodes which have a much lower forward volts drop, of approximately 0.2V, giving me more power to switch the FET. If you have some germanium point contact diodes, like those used in crystal receivers, will work just as well.

My interface uses transformers culled from my extensive scrap box, however, after a search I found some suitable replacements available from Rapid Electronics. Unlike the original interface design mine has additional flying leads enabling the use of  Notebook PC in place of the Tablet - the number of digimodes available for Android is still relatively limited, so I like to use FLdigi and MMSSTV as well.

Just as a final note, if you see any old PC keyboards floating around at the tip or a junk sale, the type with a light green PS2 plug, salvage the cable with plug, as it can sometimes be used to connect the interface to the data socket on the FT-817 as its the same plug - unfortunately later keyboards use a cheaper cable with the spare cores removed. :/