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 100 kHz 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 optoisolator. The phototransistor side of the optoisolator can be used in either a source or sink type logic level swing. Both type circuits are shown.