Improved limit switches

I had written last year about the need for limit switches that prevent the motor drive from going too far and of the difficulty of finding inexpensive weatherproof switches that would stand up to outdoor conditions.

Any electrically controlled machine needs limit switches. In the case of the DIY solar heater, allowing the motor to drive the mechanism too far could cause various sorts of damage.

The switches mounted last year did not handle the winter well. The rubber seals had already started to crack and one metal part had started to show rust. I'll describe the solution that I have come up with that hopefully will prove to be more long term. This is the limit switch which will be detailed in the plans.

I mounted a weatherproof standard electrical box on the drive arm to hold the switches. As the drive arm rotates, the box (and the whole array) tilts with it.

Inside the box, I mounted two mercury switches arranged at 50 degrees from the vertical, so that as the assembly tilts with the drive arm, eventually one of the two switches will cut off the motor. The motor current runs through both switches.

You can see here (click any picture to get a closer view) that the two mercury switches are fastened to a wooden plate with small screws so that their angle can be changed slightly. Two silicon diodes wired across the switches allow the current to flow in the other direction if either switch is open at the limit.

In other words, if the west limit is reached, the Red Rock solar sensor can tell the motor to go east, but no further west.

Mercury switches are interesting devices. They contain a small drop of the metal mercury which is enclosed in a sealed glass vial along with two (or sometimes three) contacts. Since the mercury is conductive, when it sits in the end of the vial with the contacts, electricity will flow between the two contacts. When the vial is tilted so that the mercury flows to the other end, electricity cannot flow.

Mercury switches are capable of handling relatively large electrical currents, more than the one amp drawn by the motor used here. Since the vials are sealed and presumably in a vacuum, there is no oxidation or contact deterioration if there is a spark when the current starts to flow or when it is interrupted. Because they are sealed, they make ideal switches for outdoor use in this application.

Mercury switches are commonly used (or were commonly used) in room temperature thermostats and in auto applications such as turning on an under hood light when you raise the lid of the engine compartment. Mercury switches have a bit of a bad rap these days because the mercury is hazardous to the environment if it escapes. So this is in fact a good use for old mercury switches. I got mine from an old Honeywell commercial thermostat which actually contained four of these switches. I've since found a commercial source for the switches but they don't have the nice mounting bracket that the scavenged switches came with.


Here is the limit switch assembly being bench tested. I had clamped the motor drive vertically in a bench vise and I drove the motor with a bench power supply so that I could adjust the limit switches to cut off the motor in each direction at 50 degrees.

You can see that in the right switch the blob of mercury has moved to the left end of the vial, thus pulling away from the contacts at the right end of the vial. The motor drive was rotating the drive arm counterclockwise but now cannot go any further because the motor current which goes through the switch is now cut off.

There will be more detail and a circuit explanation in the plans, but for now, if you want to know more, you can visit the Red Rock website (www.redrok.com) to see how Duane has the circuit arranged.

Here is the finished limit switch assembly ready to be mounted on the drive arm.

It has been working well in the array for about two months now. After the initial bench checkout and a further test run on the array, I haven't had to worry about it or adjust it or even open it up. It has been very reliable.

As for all the components of the DIY solar heater of course, the long term performance in our Canadian climate will be the best test of all.