A mercury tilt switch - with hysteresis

[Weldingrod1]

Old school mercury thermostats used the weight of the mercury shifting to add the hysteresis.

I think you could do the same.

OMG don't heat a mercury switch to glass melting temps! Buy what you need!

Another thought is something like a sump pump on/off switch actuator by a collar on the float rod.

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[markba633csi]

There are small microswitches that actuate with very light force

 

[kb58]

I ordered a couple microswitches with long levers that can be lengthened if needed. The idea is that a long flexible lever arm can provide the hysteresis.

 

[tq60]

Or...use 2 with a relay.

Use magnetic Reed switches with the magnets they use in pocket screwdrivers.

Reed switches come in SPDP, use those.


Place one where motor is to START.

Place other where motor should STOP.

Use a 2P2P relay in latching mode, first Reed switch makes contact to turn relay on.

Second spdp is in series with relay latch, when gear gets to stop position the Reed opens and stops the relay.

All are tiny and being magnetic control in wood gears, can be inside and out of sight.

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[kb58]

Yes, the relay and reed switch solution is next up the complexity tree if the microswitch doesn't work out.

I keep thinking about the suggestion to have the mercury switch itself rock to provide hysteresis: it's a good idea but seems tricky because wires are attached to it, with the stiffness likely to override the balance. I'm curious how it's set up in a wall thermostat to avoid that.

 

[Firstram]

Yes, the relay and reed switch solution is next up the complexity tree if the microswitch doesn't work out.

I keep thinking about the suggestion to have the mercury switch itself rock to provide hysteresis: it's a good idea but seems tricky because wires are attached to it, with the stiffness likely to override the balance. I'm curious how it's set up in a wall thermostat to avoid that.

I think the bi-metallic spring it’s mounted to is so soft and springy the slop dictates the hysteresis. Think being the operative word!

 

[Truckmannorth]

I have been following this thread as I find everything to do with clocks interesting. You are correct that the switch is connected to a very soft spring giving which dictates the hysteresis. See the attached photo.

 

[jmkasunich]

I have been following this thread as I find everything to do with clocks interesting. You are correct that the switch is connected to a very soft spring giving which dictates the hysteresis. See the attached photo.

The "spring" is also the bimetallic strip that senses the temperature. Very clever design, no wonder it's been around for 70 years.

 

[kb58]

Ah, that pic is very helpful in understanding how it works. So, it's attached to the outer end of the helical bimetallic strip... well of course it is since it has to be. The wires appear to head toward the center, minimizing the torque, also makes sense.

Anyway, I spot-welded a longer arm (shim stock) onto the microswitch, then messed about (I believe the Brits call it "fettling") until realizing that switch hysteresis would be maximized with the switch at the outboard end, with the lever triggered by an arm near the pivot. Attached the switch, added a triggering pin, and the result is very acceptable, with the arm moving about an inch and a half during activation. No more mechanical "stuttering!" Thanks for putting up with my diatribe. Picture is before things were neatened up.