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 you can use to sense mechanical rotational position. This is especially valuable for outdoor projects since the switches are sealed. I am intending to use these for low voltage (about 12 volts) control where the switched current won't be more than an amp.

I am replacing 80s technology with recycled 90's tech. At my house, I have replaced the existing 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. According to the manual: this thermostat is actually a small but powerful computer. 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. The black thing with two wires hanging out the left side looks and feels like a battery. Is it possible that is a DC operated clock?

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.

SAFETY REMINDER: Mercury is toxic and we must take a lot of care NOT TO BREAK THE VIAL and let the mercury escape. It is best to work over a pan so that should the vial be damaged, the mercury will escape into the pan. Keep a pill bottle handy for any escaped mercury metal.

In North America, you are more likely to see round Honeywell thermostats, like the T87 (complete data sheet here). It comes apart easily by undoing the single long screw that holds the adjustment post in place.

Recently, I rescued three T87 from a lighthouse that had new heating installed. Thank you Cabot Head. Three is ideal since if I have trouble with any one I will still have the two I need.

When I removed these from the wall, I did not know that there were three screws to remove the dome from its base so some damage resulted, mostly to a metal wiring terminal post but that won't matter for removal of the mercury switch.

Here is the T87 dis-assembled with the mercury switch on its thermal spring. The wire terminals are clean and undamaged. The drop of mercury is in the glass vial which is not chipped or cracked. With this one, the glass vial can be separated from the mounting clip, it just slides out the fingers, they haven't used any glue. But I think that we can make use of the clip assembly to mount our mercury switch.

Could we use the whole tilt-able mount assembly to have a fine adjustment built in to our own mount? For what I have in mind I don't want temperature to be a factor so I would like to replace all or most of the bi-metal spring which is the sensing element for temperature in this thermostat. Newer ones use a thermistor or a semiconductor sensing element and they don't contain a mercury switch.

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

Mercury switches (in the box at the red arrow) are used to limit the rotation of the array in the pic of a parabolic solar collector.

Two mercury switches cut off motor current when the array tilts over as far as it should go. To go further will cause metal parts to jam together. Stalling the motor uses a lot of power uselessly so it is best to avoid hitting the mechanical limit. The mercury switches help prevent that.

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

George Plhak
Lion's Head, Ontario

solar bbq

This home built solar cooker is a parabolic solar reflector with a spit rod at the focus. A simple aligner post at the bottom front shows clearly when it is perfectly aimed at the sun. Aiming seems not that critical and the post makes it easy. We checked and moved very slightly only once during 1/2 hour of cooking.

Today was the first time I was able to try it. For my test, I cooked one hot dog to perfection. The test was done again the next day with four hot dogs. I would like to do a non-meat test.

Two things weren't perfect: I'd started cooking in the late afternoon, at about 4pm as the sun was already part way down the sky. Also, the reflective surface is not a mirror finish but shiny aluminum roofing flashing as it comes off the roll which might give I think about 1/2 performance, yet a useful test in itself. In spite of these two factors, I had a steaming hot dog (160F) in about a half an hour. I was hoping to be able to grill somewhat (brown the surface) but that didn't happen, it did not get hot enough. I will use a mirror film shortly and start cooking earlier in the day.

I can heat small objects which skewer onto the thin stainless rod which slides into a slot along the focal line. I used a round stainless rod but a square, flat, oval or triangular cross section might work better since the payload (what's on the spit/skewer) has a tendency to slide around on the round road as it cooks and shrinks.

The characteristics of the surface of what I put on the spit will determine how it heats. I thought that this might be useful for marshmallows but their bright white color makes it almost certain that they will reflect away most of the heat.

The aiming is manual. The small disk with a perpendicular dowel makes a sensitive indicator of the sun's position. If the shadow is small and falls almost directly near the dowel, the aim is good and the sun's concentration achieved is about 18 times.

click any picture to enlarge

Aiming disk. The shadow of the small dowel on the disk shows less than perfect alignment and gives an indication of which way the bbq should be turned in order to point more directly at the sun. In practice, the aiming is not super critical. If the shadow is within about 5cm of the dowel, this is good enough. We moved the trough only once in the half hour and probably didn't need to do that.

View of the rear. I am using a video camera tripod which makes an excellent mount. Not a polar mount but it gets the job done. I have attached a small plate with a captured nut the same as on the bottom of cameras (1/4-20) so I attach the bbq to the stand with one screw, the regular one used to attach a camera to the stand.

Trying again today with four dogs and starting mid-day. My assistant Justin is showing how bright it is standing close to the reflector. We have put a strip of aluminum foil over the bottom third (it is not very smooth) to see if it was more reflective than the aluminum roof flashing (if dogs heat quicker over the foil) and possibly to catch any drips.

One of the dogs after about 25 minutes. The glisten makes it difficult to measure the temperature with an optical temperature meter, there is scatter depending on how it was pointed. Done as before by removing the spit from the bbq and moving it to a shady area to measure.

It is hottest closest to the spit. It would not be effective, for example to thread the hot dogs onto the spit through their radial center as opposed to lengthwise, through their axial center even though it might seem to increase the capacity of the bbq (more hot dogs cooked at once?).
With the aluminum roll flashing as the reflector, I estimate about 200 watts of cooking power available. This could possibly reach 400 watts with a mirror reflector.

The finished product. Could be used also for kebabs or other long, narrow items.Temperatures as high as 160F to a low of about 70F so I don't have faith in the measurement. The dogs were warm to the touch and mouth though and already cooked (according to the package). I would still like to broil them so need to get mirror material for the reflector.

Still, I am encouraged to see that this works. If you only had access to roof flashing and a bit of plywood, some straight angle metal (aluminum, steel, plastic, fiberglass etc), a workable spit bbq can be achieved at very low cost. The cost of the materials as I did it (not including the tripod) is about Cdn$10-$25. Using flexible mirror in place of the roof flashing might double cost but improve performance, probably near double (will cook faster and hotter).

This is an example of what can be done with the gen2 design in a simplified version. The gen2 book is available here in either downloadable or paper version.

clothes dryer heat recovery

This easily built project recovers warm moist air from a clothes dryer that would otherwise be exhausted to the sub-zero outside.

Energy is saved two ways: heat used to dry clothes remains in the house and less cold air is drawn in by preventing negative pressure. Electrical energy that is used to dry clothes contributes to house warming. Moisture that is added to the house air is welcome during a northern winter when the cold temperature outside causes the air in the house to become very dry.

I only use this in winter. In the summer, I re-route the vent to the outside but I use the clothes dryer only on rainy days. I use the outdoor clothes line whenever possible. I also have a indoor clothes line which is used on damp days and even in the winter.

My clothes dryer is used as little as possible as it uses an enormous amount of electrical energy. The nameplate on mine lists 25 amps at 240 volts or (25x240=) 6000 watts. When I do use it, it is good to know that the expensive heat is used twice by not being blown outside.

I made a simple box of scrap wood with a slot in one side to insert and remove a standard furnace air filter. The size of the box is determined by the size of the furnace filter. In my case, I used the same filters as my heating furnace so the inside dimensions of the box are 20x20 inches by about four inches deep. The slot is 1 inch wide to accommodate the filter. See the video below for more details.

The box has wood battens inside along three sides to guide and support the filter. On the side opposite the slot, facing downward, a right angle 4 inch heater duct fitting is attached to feed air from the dryer into the box.

The box has a plywood back and is attached to the wall above the dryer with two wood screws. Heated moist air comes into the box through a flexible aluminum pipe attached to the duct and exits through the filter into the room.

The entire box is primed and painted with ordinary house latex (water based) paint for moisture protection.

Installation involved removing the existing aluminum flex hose from the wall and attaching it to the duct on the bottom of the box.

I then filled the existing exhaust hole through the wall with fiberglass insulation.

As I did this I thought about what a heat loss this hole represented with it's aluminum tube to the outside wall. Another benefit of the project is to effectively seal up this leak to the outside for the winter.

This is the existing flex and rigid pipe as removed from the dryer after re-attachment to the bottom duct of the heat recovery box instead of to the outside vent. The dryer was then plugged in and the hose attached before sliding it into place.

Some of you with larger families who live in climates with warm, damp winters may prefer an approach using a heat exchanger to remove the moisture from the air before it is released into the house. Here are two approaches: one by Gary Reysa at builditsolar and another by ALK Engineering. I think that you will agree that these are quite a bit more complex than the my simple approach but as I said, I welcome the moisture and don't have very much of it. I can tell if I have excess moisture easily when I see condensation on the windows in the same room. With one load at full heat with -15C outside, the moisture from the clothes dryer dissipates throughout the house and does not normally condense on the cold window.

You can buy commercial heat recovery units at the Home Centers like this one from Jafine.

Simple, like mine, this is a plastic box with a diverter valve and a nylon sock to catch the lint. I have used one of these in another home and it does work.

It's a pretty cheap product though and messy to clean. I spent less money on my home made project. I think mine looks and works better and will probably last longer.

Please watch this video showing some views of my project.



Thank you for your interest.

George Plhak
Lion's Head, Ontario, Canada

Update June 20 2019:

What it looks like when the filter gets changed.

The old filter slides out the side. A bit of work with a vacuum cleaner to clean up the box. This is after about a year.

Carry the filter outside as gently as you can to not disturb all the fluff.

Amazing how much lint is caught even after the lint filter in the appliance. Normally this would be vented outside or clogged in the outlet pipe.

I fold them over an put in the garbage. I don't think these are recyclable?