Silo monitoring project

Thinking out loud, stop me if I’ve stepped off a cliff:

The HAT’s GND is almost certainly USB DC V-, while your sensor’s ground ref is probably its power V-. So the safest alternative seems to be what I mentioned before: Power both sensor and Pi from the same source, stepping down 12V DC to 5V for the Pi via a buck converter or something. That way, both devices should have the same signal ref, and everyone’s happy.

If that’s not possible, then I think you’d have to run the sensor output through an opto-isolator or something, so that the correct signal voltage is seen on the HAT even though there’s no common signal ground.

If the Pi’s Ground pin is 0 V DC (which it is) and the sensors ground pin is 0 V DC, there is no harm in connecting the two V- / ground pins together. Even if each device has a different V+. The only real concern is feeding a voltage into the hat inputs that are outside of its acceptable range. Anything up to +24V DC is fine.

What neutral wire? What board?

Something to keep in mind, even though there are contacts on the Pi and hat marked GND, more often than not they don’t go to Ground, not earth ground anyway. Not unless you connect it to earth ground. And more often than not, there is no need to do that. It’s just a common reference. V- which is zero volts. They could have just as easily marked those pins as - . As that’s all they are connected to, the - terminal in the power supply. The official Pi power supply is two pin, it doesn’t use the third ground pin, as its not needed. And the - “isn’t” connected to the AC neutral pin. Not on the US North American supplies. The AC plug terminals aren’t keyed. Either pin can be hot or neutral depending on if its plugged in up or down.

The question is for in general and for the above-listed VMX01 planning.

The pressure switch is basically just a switch.
C is common,
NC is Normally Closed
NO is Normally Open.
I would think when not activated by pressure the C is connected to the NC contact and the NO is an open circuit.
And when switched C is connected to the NO contact and the NC is an open circuit…
If you connect C to the hats ground pin the NO contact will be a Low when the pressure switch activates. Set the input to look for a ground / low signal for detection.
Or wire the switch C contact to +5V and look for +5V signal for detection. Using Ground is a safer option, if you short something out no harm is done. You will just get a false signal detection.

IMHO you don’t need to go anywhere near the AC neutral wire.

My apologies for the long wait before replying. The corporate has canceled this project. Saying that they are worried that this project might lead to reverse engineering by WAM employees that is not authorized to do engineering projects. Saying they can not stop me from making the project but they will not allow me to use there equipment and technologies for the project.

And since I’m not a regestered company I do not qualify to buy the equipment since the only sell the components to companies not individuals