Adapting existing system for automation

[EyeofdaHawk]

For the outputs you may use a relay board as ZapperMike shows in the picture.
The pt100 interfaces are SPI, so they need 3 wires plus a CS por each one.
The relays could be connected to port expanders. I use i2c LCD interface to drive an 8 relay board.
They are cheap and easy to find. They are PCF8574 in deed.
DS18B20 don’t need a pin for each one. The data line will read as many as 8 or 10. Depends of the wire length. They work better as a bus and not a wire for each one from de controller.

Thanks for the advise, alas i'm still trying to understand it (yes i know i'm still way over my head, but appreciate your patience).

So for this, i only need one DS18B20?

And for the PT100's, i'd also plug them into the GPIO Screw Terminal Block Breakout Module - https://a.co/d/7WB89cq?tag=forumyield-20?

Next terrible diagram, for the valve controllers. Does this look right? Would i need 1 IN4001 for each valve? And how many MOSFETs would i need?

Also assuming I could wire this into the GPIO board?

Thinking something like this:

 

[Lalo_uy]

The mostfet are to drive the 12v motorized valves, it is not clear if yours are 12v dc or 220v ac

1n4001 are just diodes used for protecting the mostfets when switching the valves.
You will need 2 mosfet or relay per valve.

 

[ZapperMike]

My temp probes Ds18B20’s wire directly into the GPIO board they will show up with each having a unique address that shows up in the CraftBeerPi program. From there you can designate them for different equipment as well as calibration. I’m not sure if the PT100s work in the same way.

 

[Lalo_uy]

My temp probes Ds18B20’s wire directly into the GPIO board they will show up with each having a unique address that shows up in the CraftBeerPi program. From there you can designate them for different equipment as well as calibration. I’m not sure if the PT100s work in the same way.

No, the pt100 are platinum resistors of 100ohms at 0C. You need an analog amplifier and interface to reed them.
The most popular is MAX31865 boards with SPI interface, one module for each sensor.