I was waiting to see who would come up with this simple and obvious
solution! Good job!

Roger wrote:

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?

Thanks,

Use a double pole double throw switch, one site for the load and one
side for the position.


--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"

The Lost Deep Thoughts By: Jack Handey
Before a mad scientist goes mad, there's probably a time
when he's only partially mad. And this is the time when he's
going to throw his best parties.