"The Link" interface board

[David McKenzie]

In anticipation of assembling the link in some sort of "final"
configuration, I decided that in lieu of the illusive Nomic that was
originally destined for this project I would put together a plug-in board
for the RasPi that has similar features.

I have 3 copies of the board being printed (at the bargain price of $15).
It exposes seven GPIO ports for PTT and other features of the radio that
may or may not need to be controlled, and the pins from the header also
allow for "green wire" mods for analog or digital inputs if necessary.

It also has +5V/GND connection to power the raspi from an external
powersupply/voltage converter rather than USB.

There are two 3.5" stereo phone plugs, one plugs into "audio out" from the
RasPi itself, the other is for a monitor speaker if desired. Audio from the
rig to the RasPi is to connect directly to the USB sound dongle (not in the
schematic), and audio/ptt out to the rig is via an RJ45 jack with jumpers
to select which pins do what. The audio from the RasPi to the radio goes
through an isolation transformer and can be varied in level by a 10k pot. I
"borrowed" the audio circuit from the Nomic schematic (it's identical to
pretty much every other rig audio interface circuit I saw as well), so
hopefully it works out ok.
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