Sensing switch closure with & without power applied?

From: DaveC (me_at_privacy.net)
Date: 01/08/05 

Date: Fri, 7 Jan 2005 16:02:26 -0800

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, 

-- 
Please, no "Go Google this" replies. I wouldn't 
ask a question here if I hadn't done that already. 
DaveC
me@privacy.net
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