Homemade differential thermostats with arduino

What is a differential thermostat?

Be careful not to confuse it with a normal thermostat.

A normal or conventional thermostat, typically used in heating, is a thermostat that compares the temperature of 2 probes. One of them is always in the thermostat housing itself, where it takes the temperature of the environment.
That is to say, the conventional thermostat always compares the air in our room with the heating device.
In the refrigeration world, as occurs in large cold rooms and freezers, it works exactly the same way, comparing the ambient temperature with a probe of the external device.

Therefore, a differential thermostat is not just a thermostat, and it is not something that we can easily buy in our DIY store.

A differential thermostat is a device where we can install several probes, there may be 3, there may be 4, depending on the thermostat and the use that is made of them.
Each of these temperature probes is connected to the outside and none of them is exclusively used to measure the ambient temperature.

The differential thermostat is the heart of the solar heater, it is the strategy, is what determines the performance and maximum benefit of the sun.

A differential thermostat for a DHW system or solar water heater is like the engine for a car.

Known types of differential thermostats

Until the time I was developing this project (approximately around 2008 or 2009), it was not easy to get a differential thermostat as such.
I even found in the Chinese market the mention of some differential thermostats that were actually normal thermostats for heating.

I could see some installation companies designing their own differential thermostats as a controller with various functionalities.
But they were very simple devices, using analog temperature probes, and they did not offer clear information on the characteristics or the possibility of acquiring it outside of the contract for installation and service.

We know for sure that almost all commercial controllers use analog thermal resistors.
As we also know that there are people who are fond of electronics who have already made their own custom differential thermostats.

Now in the update, March 2021, that I am migrating this blog from wordpress to another platform based on Django, I have done another search.
I must say that now I can find differential thermostats on Amazon.
Although I will also say that commercial technology is not offering much in this regard, the few differential thermostats use PT100 analog probes.

Bought from a wholesaler called "Salvador Escoda"

The overall experience was not good at all.

The configuration was complex with a 3-digit screen.
In general, its interactivity was excessively slow.
The analog probes sometimes seemed to give somewhat suspicious data.
Sometimes it restarted randomly.
This differential thermostat had several models, where the variation basically consisted of the number of probes and functional relays, significantly increasing the price.

I ended up I discarded it due to its low reliability, and that's when I decided to make my own solar differential thermostat.

How I started to create my own 100% homemade solar differential thermostat

Excited by this project and totally disappointed with my commercial differential thermostat from the MundoControl brand, I started searching the internet thinking about the possibility of making it myself.

Around 2015 I found the existence of computer controllers in the Arduino brand, in which curiously you could control electronic things by programming in processing language, which is a language very similar to the C language, and I already had experience in that.
The path of this other subproject (the manufacture of the homemade solar thermostat) was very long and full of errors and complexities.

If you have basic knowledge of electronics and programming, it is not very difficult to design your own homemade solar thermostat.

You can do it with any Arduino of any model, and you can also do it with a .
If you want to build your own homemade differential thermostat on your own, and you want to try it with raspberry, either because you like it more, or because you prefer to program in python, there is a version of raspberry that It is called the Raspberry Pico.
The Raspberry Pico is a microcontroller, just like the Arduino. Therefore, it does not need an operating system, it works similar to the Arduinos.
The Raspberry Pico is a very interesting option, as well as being much more powerful than any Arduino, and slightly cheaper.

There was so little information and the acquisition of a differential thermostat, that I began to expose this small project on a website whose domain was termostatodiferencial.com.
People began to ask me to make one for them, and I began I started a very small business where I made and sold homemade differential thermostats on request.

As I have another website dedicated to this, I am not going to go into more detail on the subject.

Those interested can take a look at http://termostatodiferencial.com

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Homemade differential thermostats with arduino