[GreenKeys] 18 volt Internal Loop Supply

[Harold Hallikainen]

On reverse polarity for motor start/stop, when in high school, my friend
and I ran a mile of wire through trees between our houses. We ran TTY over
that wire using ground as the other side of the loop. We had a delayed
release current sense relay at each end and a loop supply at each end. The
loop idled with no current. Either of us could turn on a loop supply to
start both machines and leave a message.

On my current DSP TU project, I'm thinking of just using lack of loop
current as a motor shutdown. I'll have a loop patch box (similar to that
at the top of https://w6iwi.org/rtty/) with a bunch of shorting 1/4 inch
TRS jacks, an internal loop supply, a slow release SSR (resistor in series
with loop, diode to SSR input with capacitor across SSR input), and an AC
outlet. When there is loop current, the AC outlet is active. If there is
an interruption to loop current (maybe 200 ms), the outlet would turn off,
stopping the motor. This puts less I/O on the TU.

Anyway, that's what I'm currently thingking.

Harold
https://w6iwi.org/rtty/DspTU/