On Thu, Mar 21, 2024 at 8:42 AM o1bigtenor via Dng <dng@???> wrote:
> Some would consider what I want to set up as IoT, some might suggest using SCADA some would suggest a web based system - - - but I'm wondering if I could be served by something else.

Sometimes it helps to look at similar working systems.

You are reinventing the PLC. Codesys is a commercial soft-PLC that
runs on Debian, quite cheap and affordable on a raspi, I've messed
around with this stuff and it works well. Even if you end up not
using codesys if you're reinventing the concept of a PLC this would
give you some insight into what you're trying to invent.

One system-level suggestion I'd make is the cost of a pi or three is
pretty low compared to the cost of the entire system's capex and opex
so you're probably in territory where you have like three separate
implementations. One's a literal electronic humidistat and fan,
another third of the sensors and fans are on a "real PLC" like codesys
and another third of the sensors and fans are on I donno some homemade
Node-RED thing or homeassistant automation or a big ole Python script.

Also, you can have the systems monitor each other with available GPIO.
Nothing wrong with the PLC "keeping an eye on" the behavior of the
other two systems.

You might even want to break the sensors/fans into quarters and have
one system be as simple as a timer, no sensors at all.

"All hands on deck" everything has to row in the same direction
probably statistically never happens but complete system failure might
be very bad, so having multiple control systems would help overall
reliability.

Another systems-level suggestion is you have four separate problems,
acting, logging what it did, and monitoring/reporting unusual short
term conditions and long term trends. It's likely you need four
separate "systems" not one monolith that tries to do it all. So far
only talking about the "acting" PLC solution.