[GreenKeys] Loop interfacing

[Jim Haynes]

Don asked me to comment on the suggestion of using a capacitor in
the loop to speed up the circuit, similar to what is used to speed
up relay operation.  The capacitor applies full voltage to the
magnet, and then a series resistor limits the current after the
capacitor discharges.

That will probably work; but I think there's more likelihood of
success with a circuit that uses a transistor to make a constant-
current circuit.  This is basically what is done with that complicated
selector magnet driver circuit in Models 32/33/35.

There was an article "The Modern Teleprinter Local Loop" by Frank Merritt
VE7AFJ published in QST for January 1972, page 40.  The author cites an
even earlier circuit by Don Stoner W6TNS that was published in RTTY
in November 1965.  Merritt's circuit uses 40 volts, and Stoner's 24
volts.

Another article that turned up while I was looking for the above is
"A Universal RTTY Current-Loop Interface" by Stan Nafziger KF4BY
in QST for January 1984, page 17.  This circuit uses an optoisolator
controlling a transistor.  This author does not use a low-voltage
loop, but the transistor base current is taken from collector voltage,
so the transistor is not operating in saturation.  Someone who is less
lazy than I am, or who knows how to do circuit simulations, might
look at this to see if it gives faster response than the saturated
transistor approach and might allow operation at lower loop voltage.

Jim W6JVE


jhhaynes at earthlink dot net