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I sketched out my main installation in my project vehicle. To summarize, it runs with isolated DC converters, one gets the input from main battery (standard 12V lead acid) and converts it to clean stable 12V. This 12V is used on a high side switch that switches 2 loads. Another buck converter is 12 to 5V one, since stable clean 5V is needed for the three microcontrollers.

My problem is I'm not sure what would be the proper way to ground this setup. The load1 and load2 will most likely be grounded to chassis ground. But the Arduino 1 will use 3 buttons for typical logic level function of buttons. Where would be the best practice to ground these buttons, 5V buck converter's -Vout ?

Also - the 3 cables carrying the pin signal from Arduino will have to be at least 5 feet long, I presume shielding would be advised because of interferences of the engine and such?

Thanks for any help

enter image description here

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  • \$\begingroup\$ You seem to be missing a very fundamental point - you want your two loads to be grounded to chassis, so that means the output of your ‘buck’ ( if it is isolated, it is no longer a buck, but a flyback converter. Calling it a buck is just confusing as that implies non isolated) needs to be connected to chassis. Then your Arduino needs to connect to the high side drivers, so that means the Arduino supply needs to be connected to chassis. Any isolation has just been made recundant. Grounding to chassis is the least of your challenges. \$\endgroup\$
    – Kartman
    Commented Jan 12, 2022 at 7:33
  • \$\begingroup\$ OK - this is how I planned to wire everything up - i.imgur.com/1PUkmox.jpg Are there any glaring issues in this approach? \$\endgroup\$
    – Varonne
    Commented Jan 12, 2022 at 10:27
  • \$\begingroup\$ Yes. How is the Arduino going to operate the high side switch? \$\endgroup\$
    – Kartman
    Commented Jan 12, 2022 at 10:38
  • \$\begingroup\$ My bad - the high side switch is a shield on Arduino 1. Tested that setup on my workbench and it all works, left it on for few hours to be sure. The buck converter is from Traco and has been rated for industrial use, railways and such, I don't think my alternator and battery could really surprise him during cranking, so my other real concern is just the issue of proper grounding and noises, specifically carrying the signal from Arduino GPIO to the buttons, 3 meters away, so how do I make sure those 3 wires for 3 buttons dont get some weird interference and mess up the high/low state \$\endgroup\$
    – Varonne
    Commented Jan 12, 2022 at 10:45
  • \$\begingroup\$ @Kartman that's a shield, it's just the way it works, the shield itself has 12V input and outputs for 12V loads, the Arduino takes care of actual switching via the digital signals that you compile. \$\endgroup\$
    – Varonne
    Commented Jan 12, 2022 at 10:55

1 Answer 1

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Personally I do not like 5V signals connected to buttons etc. A simple solution is to ground one end of the button and connect a Pull up resistor in the range of 6 to 10K to the 12V battery power. enter image description here

The pull up resistor forces at least 1mA through the switch eliminating the need for a dry switch. the 51K limits transient voltage into the 74C914 which will tolerate 24V+ on its input when powered with 5V. I would suggest instead of supplying 5V to the arduinos you supply 8V via Vin. I make this suggestion because of the extra noise filtering you will get because of the onboard regulator. The electrical power in a vehicle is extremely nasty. Plan on transients in the range of 80V if an older car (load dump) and reverse battery at 24V (some road side jump start use 24V).

The output of the 74C914 is the input to the Arduino. This circuit cleans up any noise in the signal. If you want to debounce it you can place a .01 cap between pin 1 of the inverter and ground. That will save a lot of software. The converter will supply 5 volts but the additional filtering on the arduino can only help. It is not oscillations you should be worried about but transients. Automotive temperature ratings range from 85C to +150C depending where mounted. You do not have to use isolated converters, but be sure the ground is common on the input and output. There are some that is not true. Buck converters need a few more volts then they are outputting. If the input goes below that say for a 6V crank you may be lucky to get 5V out of your 5V converter.

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  • \$\begingroup\$ Appreciate your help greatly, but I don't understand few things here. Firstly, how will the digital IO pins on the arduino get their high/low signal unless being connected to Arduino and providing 5V? Also, if using industrial isolated DC DC converter with 5V output for Arduino, why would it not provide 5V to Arduino regardless of the voltage oscillations in the vehicle? \$\endgroup\$
    – Varonne
    Commented Jan 11, 2022 at 23:34
  • \$\begingroup\$ There is some good advice here but it does not really answer the question. \$\endgroup\$
    – Ryan
    Commented Jan 12, 2022 at 2:03
  • \$\begingroup\$ The 5V on the Arduino is supposedly isolated. How is this circuit going to work? There’s also no input protection - ESD is a major issue in automotive and the standards reflect this. \$\endgroup\$
    – Kartman
    Commented Jan 12, 2022 at 7:35

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