DaveC wrote:

Switch closure needs to be monitored. On one side of the switch is mains
voltage (120vac); on the other, the load.

Simple enough to monitor the switch's status: when mains voltage appears on
the load side, it's closed.

But how to monitor this switch during a power failure? I'd like to be able to
report to an alarm sensor's input (simple current loop) when this switch
closes, whether mains power is applied to the switch, or power has failed.

When power fails, I could put a simple battery and coil of a l.v. relay
across the switch. But when mains voltage reappears, removing the battery
circuit in time becomes a bit risky.

How best to accomplish this task? Ideas?

The obvious answer is an extra pole on the switch - or even replacing it
with a change-over switch.

Other ways are easy enough - depending on the impedance of the source
and load.

You can couple a simple high freqency generator to one side of the
switch and sense the signal, or lack of it, on the other. You may need
the odd ferrite bead on the wire to stop the signal going through the
supply circuit and the load circuit - rather than only through the
switch contacts.

You can fire a pulse train into one wire (capacitively or inductively
coupled, for isolation purposes) and measure either the decay constant
or the harmonic content (much the same thing) - the impedance of the
load through the closed switch will be easy to detect. Again, you may
need a ferrite bead on the supply wire to increase the supplu source
impedance so as to not over-damp things. The advantage of that is you
only need access to one side of the switch.

Also only needing one side of the switch: You can couple an rf source to
the cable and monitor its standing wave as the switch is open or
closed. With the right rf source, you could just wrap a small coil of
wire round the cable with a rectifier and capacitor to produce a dc
signal. Find the peak voltage node of the standing wave with the switch
open by sliding the coil of wire up and down the cable. With the
switch closed, the peak voltage node will be somewhere else..

Or you could glue a bit of aluminium foil to the rocker of the switch
and position an LED (for greater range, use a laser diode) and
photodiode so that they "see" a reflection of each other with the switch
in one position but not in the other. With the added advantage that you
can make the alarm go off even before the switch is altered - the
approaching finger getting in the way would do it...You could put a sign
on it ..."do not even think of operating this switch"...

If you want more ideas, let me know...

--

Sue