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There are many applications when a remote switch’s on/off status needs to be
monitored. Reed relay sensors, toggle control switches and security alarms
contacts are just a few examples. Routing a switch directly to a computer
system hundreds of feet away can often lead to erratic operation and noise
problems. If such a control switch is required, a fully isolated circuit is
strongly recommended. The circuit below provides such isolation. When
properly wired, it will have 5000 volts of isolation and can be positioned 1000
feet away.
The
circuit uses a custom ferrite toroid type transformer. About 20 turns of magnet
wire forms the primary winding and 10 turns forms the secondary winding. A
simple Schmitt trigger oscillator drives the primary winding of the transformer with a
100KHz signal. The voltage at the transformer secondary is rectified to produce
a DC voltage. That voltage is routed to the remote switch and is used to drive the LED
side of an optoisolater. The phototransistor side of the optoisolator can be
used in either a source or sink type logic level swing. Both type circuits are
shown.
Click on Drawing Below to view PDF version of Schematic
