Re: Sensing switch closure with & without power applied?

From: Rich Grise (richgrise_at_example.net)
Date: 01/09/05 

Date: Sun, 09 Jan 2005 23:04:24 GMT

On Fri, 07 Jan 2005 16:02:26 -0800, 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?
>
Do you have access to the switch leads, just raw, at the top of the well?

Then get a little battery-back-up supply - Maybe a hobby-sized lead-acid
on a float charge - and use the switch to control the coil of a 12V relay.
Use the relay contacts to control whatever you want, (like control the
pump, or not[0]) and it exactly follows the state of the switch, mains
power or no.

Good Luck!
Rich

[0] I can't help but wondering - What's the purpose of this thing? Some
kind of fail-safe?
Thanks. :-)

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