[GreenKeys] New USB to current loop driver board

[John Nagle]

I've designed and built a new board which will drive a 60mA current
loop Teletype from a USB port. This board requires no external loop
supply; it's powered entirely from the USB port.

It fits in a small case 75mm x 125mm.  There are two 1/4" phone jacks 
for keyboard and printer, a mini phone jack for a motor power control
relay (5V, 100mA max) and a micro-USB connector.  Controls are an
on/off switch and a BREAK button.  A 3-high LED indicates Power
On, Motor On, and Data to the printer.  That's it.  Output current
is regulated and will work from 55 to 220 ohm loads with no adjustments.

Steve Garrison just built up one of these boards, and it worked for
him.  So now I'm releasing the design for general use.  Here are
all the project files and documentation:

https://github.com/John-Nagle/ttyloopdriver

The Gerber files and drill files for fabrication are in

https://github.com/John-Nagle/ttyloopdriver/raw/master/board/fab/fab.zip

Send that to a board house if you want boards. I use Seeed in Shenzhen.
They charge $20 for 10 boards, plus shipping, which is about $30.

This is a moderately advanced surface mount soldering project.
You'll need the tools and skills to solder 0.5mm pitch parts.
Parts cost not including the board is about $50 per board.

Usual questions:

- Can you buy a completed board? Not from me. If someone else wants to,
it's all open source.

- Will you modify this to do ...? No, but you have all the design
files in KiCAD and can do it yourself.

				John Nagle