thermistor 15 meter wire length

Hiya!

Doing a project that has 3 thermistors, that are 15 meters away from an arduino mega board.
The wire is 26AWG, 6 separate threads together in a copper shielded sleeve.

Close to the shielded cable, are other cables that control and feed inductive charges.
The copper shield is connected to ground ( 3 meters of copper clad iron sunk in earth)

Am I going to have to protect the arduino IO ports from peak currents, using diodes and zeners and resistors etc? Or will the shielded sleeve suffice?

Please view attached fritzing diagram.

Close to the shielded cable, are other cables that control and feed inductive charges.

What does that mean?

The copper shield is connected to ground ( 3 meters of copper clad iron sunk in earth)

That’s not necessary. The shield just needs to be connected to the Arduino ground.

You shouldn’t need any special protection but you might need to add a capacitor between the Arduino input and ground to filter-out noise.

Looking at your Fritzing diagram is a waste of your time and mine. IT does not show your shielded wire and if you have grounded it at both ends(wrong) or at one end only(right). It does not show your thermistor are twisted pairs and they must be. Your ground device will serve no purpose unless it is also connected to the mains ground at the point where power comes into the building. Actually it makes things worse if your have your Arduino and it’s power connected to the mains ground.

Paul

Hummm... a Mega and a NodeMCU hung together via serial and both with rtc modules. Lots of overly complicated hardware ‘ya got there.

The first question would be why? I know you didn’t come here to defend your design but chances are it pretty good you only need the NodeMCU and a standard off the shelf analog multiplexer for the thermistors.

Happy to discuss if you want to simplify.

Oh, I also question the use of the rx/rx pins on the NodeMCU and the lack of level translation between the boards.

Hi DVDdoug, Paul_KD7HB and WattsThat, thank you for taking the time to look into this!

DVDdoug:
What does that mean?
That's not necessary. The shield just needs to be connected to the Arduino ground.

You shouldn't need any special protection but you might need to add a capacitor between the Arduino input and ground to filter-out noise.

I ment to say that I expect electromagnetic interference from power lines that feed solenoids and relays. These power lines are close to the sensor cables.
Are you talking about a low-pass capacitor filter? Could you give me a hint as to what material to read up on, so as I may define the capacitance needed?

Paul_KD7HB:
Looking at your Fritzing diagram is a waste of your time and mine. IT does not show your shielded wire and if you have grounded it at both ends(wrong) or at one end only(right). It does not show your thermistor are twisted pairs and they must be. Your ground device will serve no purpose unless it is also connected to the mains ground at the point where power comes into the building. Actually it makes things worse if your have your Arduino and it's power connected to the mains ground.

Sorry for wasting your time in my attempt at a fritzing diagram. I´ll connect one end of the shield to the arduino ground, then. The shielded cable with 6 wires, aren´t twisted pairs as you see in CAT5 cable. The 6 wires are twisted, but not in pairs of 2. Also, the thermistors share a common ground, so I´m unsure how to twist pairs. You recon it´s better to get CAT5 cable, than use this shielded 6-strand cable?

WattsThat:
Hummm... a Mega and a NodeMCU hung together via serial and both with rtc modules. Lots of overly complicated hardware ‘ya got there.

The first question would be why? I know you didn’t come here to defend your design but chances are it pretty good you only need the NodeMCU and a standard off the shelf analog multiplexer for the thermistors.

Happy to discuss if you want to simplify.

Oh, I also question the use of the rx/rx pins on the NodeMCU and the lack of level translation between the boards.

I did come here to improve on design, don´t really see it as defending haha :slight_smile: I did go with the multiplexing originally, and I did get pretty far using only the nodeMCU. But eventually I decided that it was worth my time to do the serial connection between the 2 boards, because of nodeMCU limitations on libraries(thermistor libs and DST lib). I have a few other arduinos running, they are not connected to the internet, and I hope to recycle this serial connection code, with a nodeMCU unit, to bring the other boards online as well.
I would love to ditch one of the RTCs! But I read some stuff about slave/master/rtc usage over various boards that I didn´t follow at all, and it also said it was asking for trouble.
So now I intend to have the nodeMCU RTC talk to NTP time servers, then relay the time through serial every so often, to the arduino, and write the time in the arduino´s own RTC. If I don´t use an RTC in the arduino, I´d have to send the time by serial to the arduino almost every cycle, right? You figure that might be more efficient?

You wrote: "I ment to say that I expect electromagnetic interference from power lines that feed solenoids and relays. These power lines are close to the sensor cables.".

That is the reason to use only twisted pairs inside your shielded cable. Usually a signal wire and a ground wire are twisted together.

Paul