Silo monitoring project

“If” the earth ground is a separate ground connection (only grounds the case) from the signal ground, you want to use the signal ground.

Here in Canada on any appliance that has a three prong plug, that third prong goes to ground, earth ground. And in the fuse box, the neutral and ground are connected together. You can debate if that’s good or bad in reference to a lightning strike but it is what it is. I personally don’t think it will matter, your going to have dead tech anyway.

If you have an ohm meter you can check to see if the earth ground and signal ground are one and the same. They usually are. If they are the same (wired together) it doesn’t matter which one you use.

If two pieces of equipment are a fair distance apart, you can have different earth ground potentials. I do believe its referred to as a ground loop. Something like that. It’s 6AM here and I only just got up. Haven’t had my fist cup of java.
Anyway, if you just reference to earth ground and only run the one signal wire, you can end up with issues. Run a second wire linking signal ground from both devices and things will be much better. They have a common reference that doesn’t change.

The earth ground is IMHO, there to protect you, not the equipment. Especially with mains AC powered devices. If something goes wrong inside and the live feed contacts something its not supposed to, it gets shorted to ground and trips the breaker or blows the fuse.

If I got something wrong there feel free to correct me. I’ve been out of the loop (retired) for a fair number of years and my memory isn’t what it used to be. ;)

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.

Thank you noted.

I’m attaching a note here for the Plant overlay.
Every component must be set up except for the SP2, SC - TOUCH and SC-HUB.

Note the Audible Alarm will be replaced using a 24V AC version.

New one is for a basic Pressure switch for Air Pressure.
Item Code is :VMX01

Note: Maybe I understand electronics completely wrong but shouldn’t the Neutral wire be reconnect to the power supply at some point?
I know this might be a dumb question but there is no connector for this board to go back to Neutral.
Sorry for the long wait between post.
When I have time ill update the ILT and IPE that was discussed above with what we discussed about ground.

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.