Hack: microswitch on side of receptacle, tongue bent in 'L', short side of L
pokes partially into receptacle. This will sense machanically. Alternately drill
hole and have microswitch lever (plastic) protrude into receptacle. Or provide a
cover-like thing that must be moved aside to use a recepacle. None of these
sense the circuit proper.

To sense high and low loads I have used two antiparallel diodes before. These
create 0.5-1V of drop when there is something 'in'. This can be sensed in many
ways, including with a 1:1 transformer (which will not be a current transformer
in this case). The diodes may need heatsinking, but not for low loads (a few
amps will be ok w/o heatsink, the diodes run hot). Shunt the diodes with 0.1uF
25V to avoid parasitics. The voltage that appears on the diodes is almost square
wave and will pass a 1:1 pulse transformer that will not let 50/60Hz through.
The pulse transformer must be coupled with a capacitor to avoid heating the
coil. In theory adding a NTC in parallel with the diodes can increase the load
capacity w/o heatsinking. The NTC will take over more and more current as the
diodes heat up. Of course this needs a lot of testing.

Peter P.


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