Sensing switch closure with & without power applied?

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,

Reply to
DaveC
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So have a relay on the hot side of the switch. When mains power is present, the relay is closed. When absent, it is open.

Reply to
larwe

Hi Dave,

One method might be to generate a small AC signal on an ISM frequency such as

13.56 Mhz and couple it through a small transformer to the two switch contacts. An HV capacitor can be used to prevent mains current through the transformer. Then monitor the current into that transformer. If the switch is closed there will be full current no matter what the power grid status. When it is open, the current will be a lot less. This can be pulsed if battery operated, to conserve battery power.

A small choke on one side of the switch prevents load or line conditions from shorting out your RF signal while the switch is open. Or course, you have to abide by EMC rules and obey electrical safety rules (isolation etc.). IOW, you really have to know what you are doing and be very familiar with safety and other legal regulations before doing any of this.

Also, make sure the pulses from the switch action don't fry anything.

Regards, Joerg

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Reply to
Joerg

Maybe something like this: 20K 20K ___ ___ +---|___|-------+--+----|___|-Vdd | | | | _/ | | o------+--+ +----o/ o-+-------------- A | | | | | | V - | | | | - ^ | | | +------ B | | | | | | +--+---------------+ .-. | | 120VAC | | | | | | V - Load '-' - ^ | | | o------------------------------------+----+--+-------

If there is 120VAC on A, then the switch is closed

If there is not 120VAC on A, but (say) > 300mVDC on B, then the switch is open. You might have to shunt the load with a resistor and perhaps reduce the mV comparison level if the load doesn't look like a relatively low resistance.

It reduces the load voltage by one diode drop and the diodes on the left have to be rated for the load current. The 20K on the left has to be rated for any reasonable line voltage transients.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

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...

Reply to
Palindr☻me

On Fri, 7 Jan 2005 17:09:10 -0800, Palindr?me wrote (in article ):

It's a float-operated microswitch in a well.

Reply to
DaveC

Perhaps isolating the float and use it to drive a relay with a couple of sets of contacts from a backup battery ? Depends on how important it is for ya I guess.

-- Regards ..... Rheilly Phoull

Reply to
Rheilly Phoull

Turn the switch into a relay and a momentary contact switch to energize it (with a feedback connection to keep it energized). Then you know that if the power has failed, the relay is open. In other words, instead of finding a way to answer the question, avoid having to ask it.

Reply to
CJT

In that case, use it for low voltage signaling with a battery backup. I wouldn't put mains voltages down a well, anyway.

Reply to
CJT

Single pole changeover? Thinks ....

Reply to
budgie

Then it should have NO and NC contacts and you can wire up your relay and battery to the contact pair which never have mains across them.

Reply to
nospam

LOL, well, you won't need the sign then - unless you work=20 for the Mafia.

As well as my original, rather more serious suggestion,=20 echoed by other posters, of using change-over contacts and=20 the very sensible suggestion of using the switch to control=20 a mains relay - which can have auxiliary contacts, if need=20 be, you might want to consider:

Stick another float-switch down the well and wire that to=20 you alarm circuitry.

Not only does that give electrical isolation, it also means=20 that you have a fail-safe - should the existing switch fail=20 or get jammed. This can be invaluable.

Presumably you are worried about too high well levels (it=20 happens here in the UK with some wells in cellars of old=20 houses - unless you start pumping, you end up with a full=20 cellar, which plays havoc with the port). In which case a=20 fail safe alarm that says the well isn't being pumped down=20 could be invaluable. You can set the alarm switch point a=20 little higher than the pump switch float, to save heart=20 attacks..

Or are worried about "pumping" an empty well (which also=20 happens in the UK) and thus wrecking the pump? Presumably=20 not, as not much pumping is going to happen anyway, with no=20 power.

On both cases an independent water level sensor can be a=20 very good idea.

I have a mercury tilt switch on my water system, which=20 directly switches the pump motor, taking water from a well=20 in the garden. It has a big float with a piece of cord=20 running over a pulley to operate the switch (thus keeping=20 the mercury-filled switch far away from the drinking water). These mercury switches seem to go on and on for ever.=20 Your post has prompted me to go look and see how it is doing=20

- it hasn't been looked at in over 20 years, to my=20 knowledge..Thanks.

--=20

Sue

Reply to
Palindr☻me

I bet it's down in the well and totally inaccessible. :)

There have been a few valid suggestions.

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Reply to
Sam Goldwasser

Simple enough to do in practice- but the environment can make things "sticky"- be sure your sensor input is isolated or else fuse it:

View in a fixed-width font such as Courier.

+-------------------------------+ | | +----DPDT---+ +-----------------+ | | | | | | | |NC| | | | | | o COM | | level sw | | | \ o-----|--------o o------+ | | LINE>-+-------|--o | | \ | | | | |NO | | ^ | | | | | | | | | | | | | | | --- | | | | | +--------|--+ | | | | | | | | | | | | | | | |NC| | - - | | | | | o COM | | | | | | \ o-----|-----------------+ | | | +-|--o | | | | | |NO | | | | | | | | | | +-|-----------|-----------------+ | | | | | | | | | | ====== | | | | +-------|---UUUUUU--|--+ | | | +-----------+ | | | | +----------------------+ | | | | | | | | +----+ | | | NEUT>-+------------+LOAD+-------------------+ | | | +----+ | | | | | | | | | | | | | | | | lvl sw monitor relay | | | | +--------+ | | | | | ====== | | | | +----------|-UUUUUU-|-----------------+ | | | | | | | / | | | +--|--o o---|--------------------+------->

| | NO | | | +--------+ | to sensor | | +----------------------------------+----->

Reply to
Fred Bloggs

Nah- it's got to have "wahrs" coming back up to the surface.

Give it time- some homebrew lamebrain has yet to insist that a gas powered backup generator is the *ONLY* way to handle this.

Your signature file is way too brief......more reading material, please.

Reply to
Fred Bloggs

There are only a few hundred million submerged well pumps operating off

240VAC mains power, and they seem to be doing just fine.
Reply to
Fred Bloggs

Yes, but what I was trying to say is that all those suggestions saying "replace the switch with such and such" might not be viable.

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Reply to
Sam Goldwasser

Fred Bloggs wrote:

You can do a better job of preserving the level sw contacts like so: View in a fixed-width font such as Courier.

. . . +---------------------------------------+ . | +-----------------------------------+ | . | | | | . | | +----DPDT---+ | | . | | | | level sw | | . | | |NC | +---------+ | | . | +--|--o COM | | | | | . | | \ o-----|----|---o o---|--+ | | . LINE>-----+-------|--o | | \ | | | | . | | |NO | | ^ | | | | . | | | | | | | | | | . | | | | | --- | | | | . | | | | | | | | | | | . | | | | | - - | | | | . | | |NC | +---------+ | | | . | +----|--o COM | | | | . | | \ o-----|-----------------+ | | . | +-|--o | | | . | | |NO | | | . | | | | | | . | | | | | | . | | | | | | . | | | ====== | | | . +-------|---UUUUUU--|---+ | | . | | +-----------+ | | | . | | | | | . | | | | | . | +-----------------|------+ | | . | | | | | . | | | | | . +---------------------------+ | | | . | | | | | . | | +----+ | | | . NEUT>-+----------------+LOAD+-----+ | | | . | | +----+ | | | | . | | | | | | . | | | | | | . | | lvl sw | | | | . | | monitor relay | | | | . | | +--------+ | | | | . | | | ====== | | | | | . +--------------|-UUUUUU-|----------+ | | . | | | | | | . | | / | | | | . +----------|--o o---|---+ | | . | NO | | | . | | | | . | / | | | . +--|--o o---|--------------------+------->

. | | NO | | . | +--------+ | to sensor . | | . +----------------------------------+----->

. .

DPDT is low power, lvl sw monitor relay coil is only load

-or- View in a fixed-width font such as Courier.

. . . +---------------------------------------+ . | +-----------------------------------+ | . | | | | . | | +----DPDT---+ | | . | | | | level sw | | . | | |NC | +---------+ | | . | +--|--o COM | | | | | . | | \ o-----|----|---o o---|--+ | | . LINE>-----+-------|--o | | \ | | | | . | | |NO | | ^ | | | | . | | | | | | | | | | . | | | | | --- | | | | . | | | | | | | | | | | . | | | | | - - | | | | . | | |NC | +---------+ | | | . | +----|--o COM | | | | . | | \ o-----|-----------------+ | | . | +-|--o | | | . | | |NO | | | . | | | | | | . | | | | | | . | | | | | | . | | | ====== | | | . +-------|---UUUUUU--|---+ | | . | | +-----------+ | | | . | | | | | . | +-----------------|------+ | | . | | | | | . +---------------------------+ | | | . | | | | | . | +-----------------------+ | | | . | | | | | . | |-+------| |---+---|LOAD|-----+ | | | . | |>| | +----+ | | | . | | | | | | . +--[1K]-+ +----[1K]------+ | | | . | | | | | | . | | | | | | . | | | | | | . | | lvl sw | | | | . | | monitor relay | | | | . | | +--------+ | | | | . | | | ====== | | | | | . +--------------|-UUUUUU-|----------+ | | . | | | | | | . | | / | | | | . +------|--o o---|---+ | | . | NO | | | . | | | | . | / | | | . +------|--o o---|--------------------+------->

. | | NO | | . | +--------+ | to sensor . | | . +--------------------------------------+----->

. .

DPDT is low power, lvl sw monitor relay coil is only load

Reply to
Fred Bloggs

Right- you don't won't to touch the switch.

Reply to
Fred Bloggs

On Sat, 8 Jan 2005 06:22:34 -0800, Fred Bloggs wrote (in article ):

Simple and elegant. Thanks, Fred!

Your caution about isolation / fusing is in case one of the relay contacts sticks, this would put mains v. on the sensor inputs?

Thanks again,

Reply to
DaveC

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