I've been having a think about what kind of layout I'd like for my components to have on a PCB if I ever learn how to create them.

The plan is to have the Nano and Pi's USB ports available via the top/side for serial access/programming etc, with the relay and sensor outputs (and power) ins and outs via the bottom:

Arduino Nano (Blue Headers)

Either female headers or soldered directly to the PCB, connected to i2c bus (A4/A5), software serial (pins 2,3?) to ESP8266 header, relays (4,5,6,7?) and pH/one-wire sensors (use A1/A2?). 5v Power & Ground

Pi Zero (Light Green Header)

40pin Female header to allow Pi to mount over the top of the the Nano; if the Nano is mounted directly to the PCB there should be clearance to do so.

The only connections for the Pi are to 5v, GND, and i2c Bus. Full header only really required for a sturdy mount, so to speak.

Relays (Red zone)

Potentially a four channel Relay board mounted, or 4 individual relays or fets. TBC

ESP8266 (Orange Headers)

Female headers for mounting an ESP8266.

Connected via serial pins to the Arduino's Software serial pins, and also power and ground. 5v tolerant ESP module would be ideal, or tie it to the Arduino's 3.3v pin.

i2c Headers (White block)

breakout pins for 5v, Ground, SDA and SDL. For connecting LCD Screen and other modules.

1 Wire headers (Awful pinky colour)

Header for Power, GND and 1-wire data pin (A1?). To be used with temp sensors.

pH Sensor Board (Green block)

a mounting point for the standard pH sensor breakout boards with the BNC connectors.

Power, Ground and data (A0) pins to be made available for this module.

LCD Mounting holes (Black spots)

These are to insert spacers for the LCD screen.

Next step - to actually take some measurements and see if this will work!

Also need to draw up a proper pin connection list too.