Friday, October 24, 2014

weatherproof rotation switches from recycled thermostats

A good source for rotational switches can be had in old thermostats which contain a mercury switch or two.


An old thermostat contains a mercury switch which I need for projects to sense mechanical position, especially valuable for outdoor projects since the switches are sealed. I am intending to use these for low voltage control where the switched current won't be more than an amp.

I am replacing 80s technology with 90's tech. Yesterday I replaced the existing house clock motor thermostat (the clock did not work) with a "new" (to me) Honeywell/34 that had been removed from a church. The date on the "new" manual is 1994. This thermostat is actually a small but powerful computer the manual tells me. So far it is working fine. The default temperature of 20C is a bit too warm but nice as I enjoy the heat while it is not too cool outside. The furnace is not working hard yet, the outside temp is about 5C. Replacing a thermostat is best done on a day when you don't actually need the heat.

In this old Honeywell Chronotherm the single mercury switch is to the right on a tilt-able mount which is attached to the set temperature lever. Too bad we can't use the whole tilt-able mount assembly to have a fine adjustment built in to our own mount. The trouble is that the switch would change slightly with the ambient temperature and we don't want it to do that!

A closer view of the mercury switch, the small glass vial up and to the right to the right of the colored plastic post. I would like to remove the vial and it's wires without damage. I'll have to study it a bit and report back.

One possible use of these type of switches is for limit switches in a solar tracking array.

Mercury switches are used in the box at the red arrow in the pic below to limit the rotation of the array.

It is not possible to easily buy mercury switches since they have all but been banned for new construction. However by gathering these as they are being removed from service saves them from the landfill and keeps them available for a future use.

Link here goes to the motor drive of the project. The main index is diy solar parabolic trough gen2 intro

DRAFT - may change over the next day or two as I add/change - comments welcome - comments are moderated

George Plhak
Lion's Head, Ontario