> The only advantage - but a very BIG advantage - to using the
> opto-isolator is that it can be designed so there is no galvanic
> connection between the cable and the device being controlled.

Optical is a good way to go. A long long time ago I installed an
optical loop switch in a dirty and corrosive environment, made
from cheap figure-8 nylon optical fibre (1mm diameter). Light
from an LED went down one fibre and back down the other to
a photo amp. The switch was simply a plastic vane that blocked
or passed the LED's light from one fibre's end to the other. With
a little bit of tinkering it probably could have been made to work
with a single fibre. Length was a touch under 25m (50m overall)
and although intended to be used where metal wire and parts would
have disintegrated, it would have also been virtually immune to
outside interference. And is benign if faulty (ie nothing to short out
and start a fire in this case)


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