Hardware Architecture: Centralized vs. Distributed

Hardware and software are the ying and yang of what modern appliances strive to be. Evolution and oscillations of the software part of DZ is tightly coupled to choices hardware offers.

At all times, cost was the predominant factor. $99 temperature sensor is not a valid choice for someone who is most likely using a mechanical thermostat, but $3 chip probably is.

So what that it doesn't have a nice box - rich and poor are affected by extreme temperatures in strikingly similar ways, and if you can't afford to cover the chip, and you had to staple the cables to the walls instead of paying couple of thousand dollars to the crew that hides them into the walls, so be it. At least, you're comfortable now.

Same goes for professionally installed dampers vs. DIY registers.

So all right, it is clear enough that $3 chip doesn't have a human interface. All the control was initially being provided through the debug console - sure, it was limited to whoever had the password and the inclination to fiddle with it, the rest of the family had it by the way of voicing opinions and complaining (which, surprisingly, ceased pretty soon after the system was activated).

But it is crystal clear that this solution doesn't stand a chance in the long run - in order to pass the SWMBO Compliance Certification, the system must have at least rudimentary controls in each zone (provided the zone is used often enough; it is unlikely that a server room is going to require much intervention often).

Which presents quite a few interesting options.

Rudimentary controls may or may not be terminally dumb.

The dumbest possible control will include a display (two 7-segment LED is just fine) and two buttons (which actually give you more than two possible inputs: you can use "both buttons depressed" as a signal to change the control context). Plain 7-segment LEDs, though, cross the border between "cheap as in inexpensive" and "cheap as in obnoxious" - a graphical LCD won't be that expensive today, especially given the fact that it will constitute just a fraction of the total cost.

Now, what difference does it make if you have two buttons or eight? Granted, two buttons is 4 times cheaper (by themselves), but again, much less so, given the infrastructure considerations: first of all, having 8 soft keys gives you unlimited flexibility for all practical considerations, second, they won't be that much more expensive compared to the total cost.

Does the sensor have to be in the same box as the LCD and the buttons? It depends. First of all, the box has to be located where it is convenient to access it - most probably, next to the light switch. But the sensor has to be located where the actual temperature has to be measured, and that is where the complaining person is - in the bed, at the coach, next to the oven, you name it. On the other hand, the complaining persons lived with a single thermostat for all their life, so even the sensor that is in the box is already going to be an improvement. Another consideration is that this case requires less wiring (assuming it's wired; wireless solutions are a whole different can of worms).

Yet another component is the servo controller. Historically, servo controllers were always centralized - one controller, bunch of wires, servo boosters and capacitors at each servo, good enough.

Later, Nic van der Walt provided schematics for distributed 1-Wire servo controller, which would support bang/bang dampers with no problems at all, and modulating dampers that will be quite noisy - latency of 1-Wire protocol with the bunch of sensors on it is insufficient to provide software controlled quiet transition.

Today, there are chips with PWM supported in hardware, and if they have enough brains to support even the simplest quiet transition possible, then you have the distributed servo controller right there and then.

Considerations above assumed we're ignoring existing 24V dampers (which are mostly bang-bang, and regardless of whether they are or not, they're well beyond the approved spending limit for the audience) and using R/C servos to control the dampers, as shown here.

This just about concludes the story. There are three major topics left uncovered - the HVAC actuators, hardware interface selection, and making all the gadgets wireless - but the article is getting too long, so stay tuned.