Choosing proper grounds for sensitive 5V electronics (vehicle project)

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Joined Mar 25, 2021
Noob alert! I'm tinkering on my garage project, standard vehicle stuff - 12V lead acid battery, 3-phase stator, reg/rec. I would like to put few additions to its circuit. Being a rookie, I'm not sure how to approach this correctly in terms of providing safe 5V for these microcontrollers. I've read a lot of articles on transients and noises in vehicle environment - load dumps, over and undervoltages, spikes...and also proper grounding, ground loops, it just seems overwhelming hence this thread :D

I've sketched up (sorry for any drawing mistakes) the circuit I plan to make, and there's two of them, the difference being grounding of the 5V devices, since I don't know which method is actually correct, grounding 5V devices to chassis or to the buck converter providing the 5V. For the conversion I have a Traco Power TMDC 20 unit, if you have the time you can look up its specs, from my limited knowledge that converter should be good for my use case but do let me know if its not.

Also another issue I'm worried about is grounding the buttons on Arduino for the logic level signals - I presume they all should be grounded on their own Arduino GND pin - but then there's a issue of physical distance, the buttons themselves will be at least 2 meters from the Arduino, so this makes me think about interferences, noises on the wire, etc. Here's the schematic and thanks for any help :D

I am not an expert by any means, but from what I have read, experienced, and learned from seeing how vehicles are wired all "computer" grounds need to go to a common point at the "computer". From there your best bet is to connect the common ground point directly to the battery.

I would think a common ground for the switches would work, but that too should should go back to the Arduino ground. It wouldn't necessarily have to go to a ground pin on the Arduino itself, but at least somewhere between the power converter and Arduino.

One thing I found on my own project that worked out good was cutting up an old USB cable to go between two PICs. The power and ground were wired up as power and ground, and the data wires were connected to I2C pins between the two PICs. Stick with the "A" type at one end as the newer "C" cords that I have tried have smaller wires that are kind of a pain to deal with (but more of them) At over 100kHz (probably closer to 500 kHz or more... I never really calculated it out) it worked flawlessly at around four feet (a bit over one meter). Another good thing I found was some shielded 18 gauge alarm wire at the home improvement store. Granted it's only two wire cable, but it has come in handy.
The Traco datasheet lists 80 - 160 VDC for an input voltage. It also looks like it will essentially shut off if the input goes below 70VDC (Under Voltage Lockout in the input specifications).

Depending on what you are doing exactly you may be able to hack up a charger that plugs into your lighter / power outlet. Find a good one that can output the current you need, break it open, and solder some wires to it. They generally have all the clamping built in and such to deal with your basic transients and oddities. Most of the ones I've torn open where some sort of switch mode converters that were clean enough to make things work.

Edit... the datasheet I looked at
Actually you will want to make sure pins 2 and 5 are connected internally first. Normally they are, but if not then it may be possible to have a 5V output, but the +Vout could be say 12V and the -Vout would be 7V leaving a difference of 5V at the output terminals. As long as you don't connect anything after the converter to the chassis ground it won't cause a problem though.