A Better Solar Water Heater Electric Back-Up Timer

Another little problem around the house that can use a little creative circuitry to make it better. Do I really have to do it? No. I do it because I can, and because it is fun!

Solar Water Heater Back-Up Timer
A hot water heater timer setup for a solar water heater.
This time it is the electric back up heater for my solar hot water heater.

In Waikoloa solar hot water is an obvious addition to the house, we had it installed within a couple months of moving in. Considering electric power is about $0.40/kWh on the island, and tropical sunlight is quite intense, the use of solar to heat our water has been good money saving move… Long, hot showers with no guilt!

Once or twice a year we will get a period of heavy clouds and the water temperatures will fall to the point we need to turn on the electric back-up heating element in order to have that hot shower. Like most solar setups the storage tank has a electric element that will heat the water when needed.

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I took a break from working with a “big” microcontroller to work with a little one. The PIC12C671 is definitely little when compared with the PIC18F66K80 I am using in the GenPIC utility PCB. With less than 2k of program space and a mere 128 bytes of RAM it is definitely limited. Consider that the chip only has eight pins, two of which are power and ground, leaving six I/O pins to get the job done. No problem, I only need two I/O pins for this task and that few bytes of RAM is more than enough!

A simple heater using a bang-bang controller.
A simple heater using a bang-bang controller.
This project is pretty basic… A bang-bang heater controller. This simple form of controller simply turns on and off as the temperature (or some other controlled parameter) goes up and down, there is no attempt to vary the output, all or nothing. Bang on, bang off, or simply a bang-bang controller as it is known in the trade.

A bang-bang controller is inherently reliable and stable because it uses two different control setpoints, a high and a low. Because these control points are separated by a large margin, called hysteresis, the controller will not oscillate or rapidly turn on and off. In this case the heater will not turn on until the temperature falls below 10°C and will not turn back off until the temperature rises above 15°C. That five degree margin is called hysteresis, and ensures a good period of time between on and off.

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A Dew Heater Controller

I have had a problem observing here in Hawai’i. The skies can be gorgeous, there is little to no light pollution, and you have access to much more of the southern sky. What could go wrong? …Dew!

Dew Heater Electronics
A view of the interior of the handwired dew heater controller
This is a tropical island, this means tropical humidity and moisture. Dew is a feature of observing that I have not had much problem with in the past. In Arizona it was rarely an issue, but here it can quickly shut down a night’s observing. My last time out on Mauna Kea dew was a problem, it didn’t shut me down, but it was a constant hassle. I had to be quite careful about breathing on the eyepiece. One mis-aimed exhalation and that eyepiece needed to be put in the back of the vehicle to dry out!

I had one dew strap available, given to me by a friend and fellow observer, a small one just right for wrapping around an eyepiece. So I just needed a controller to get started. There are several of these available on the market. But I had some issue with the cost, it seemed a bit high for what was essentially a very simple device. Some of the commercial units offered features like temperature sensing and regulation. Simple overkill for most users actual needs, just warm the optic slightly until the dew goes away. Sounds like an excuse for another evening project to me… Build it!

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