So whats the plan?
Hook up the new one and if you also fry it then start to debug the hardware?
Your arduino is definitely gone. I agree with @jim-p though, find the issue/error before you connect and damage another one.
The main MOSFET of the 5V regulator contains a parasitic diode. A reverse current high enough may damage the regulator, possibly to a short circuit. Next time VIN is applied, it goes directly to the 5V line, destroying the processor.
So whats the plan?
Hook up the new one and if you also fry it then start to debug the hardware?
My initial idea is to check the voltages coming out of my various regulators (9V, 5V, 3.3V) and to double-check the wiring. (The previous Arduino I burned out was due to a 5V power supply that somehow burned out, and started putting 12V on the 5V bus.) If you have any suggestions, I am certainly open to them.
Thanks for the insight. I will definitely add the diode, though, as I think of my circuit (of which I WILL get a diagram up), the Vin doesnât go to any other part of ther circuit. It goes from my 9V voltage regulator to the Arduino, and that is all.
Does it work normally when connected to a computer? (no 9 volts)
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Well we really have no idea what you are trying to build. A description of your device and a schematic would be necessary for anyone here to help debug your circuit.
(The previous Arduino I burned out was due to a 5V power supply that somehow burned out, and started putting 12V on the 5V bus.)
If you circuit was properly designed that would not have caused the Arduino to burn out.
That does not sound consistent with a bad regulator on the Arduino. Did you measure the voltage on the SDA line after you disconnected it from the Arduino? If that were connected to a higher voltage it could pull up the 5V bus through the protective diode on the processor.
Iâm working on it.
Obviously, I have not described my setup well enough. My 5V source is a separate device from the Arduino. Iâll double up my efforts to get the circuit diagram posted, and everyone will have better understanding of what I have. I am busy right now, and have little time to work on it. The circuit is complicated, and will take me a little time to get it all laid out in an orderly fashion. I work from the Fritzing diagram, not the circuit diagram. The pictorial is more meaningful to me.
No hurry!
I finally got the schematic done. The 3.3V Regulator feeds the GPS chip to keep it hot, to eliminate delay at startup. The MOSFET is used to allow the speedometer to continue to run until it is able to finish its shutdown procedure. The 9V battery is there to keep the speedometer running incase of a catastrophic power loss, so that the odometer is not lost. The LED is to indicate that the battery needs to be replaced. The multitude of unconnectd jumpers are so that I can cut the circuit board in half if I need to for space.
I really can't see how that schematic can be correct.
The output of the 9V regulator goes through D6 to what appears to be the main Vcc line. The 5V regulator U4 goes to this same main Vcc line through D4, but D4 will always be reverse biased. The approximate 8.4 volts on the main Vcc line is fine going into Vin of the Nano Every, but is too high for the TB6612 Vcc input (absolute maximum 6V), and the TCA9548A (absolute maximum 7V), and is almost certainly too high for the OLED displays.
It is not apparent why you even need the TCA9548A, unless your OLED displays only allow a single I2C address.
The GPS module is operating at 3.3V, it likely is not tolerant of the direct connection to the Nano Every which operates at 5V logic levels.
The Nano Every is labeled as "Nano 33 Every", there is no such board produced by Arduino, is that a mistake, or a 3rd party board that operates at 3.3V???
Sharp eyes! Youâre right. One trace wound up atop another. I found several other problems as well. Here is the corrected diagram. I color coded the traces to make it a little easier to follow.
Both OLEDâs are the same, and have fixed I2C addresses. Hence, the need for the multiplexer.
I donât know why it was labeled as such. It was part of the Fritzing part, which I was thinking came right off the Arduino web site. At any rate, I am using the genuine Arduino Nano Every. I learned my lesson about cheap Chinese knock-offs. Only the real thing for me.
Iâll have to investigate that. I have had this module running with an Arduino, but I was powering the GPS at 5V. I have since read that it sould have been at 3.3V. I donât know enough to know the difference.
Hi, @vpeppers
What is the purpose of the potential divider R7 and R8 on the 5V pin?
Thanks.. Tom.... 
Thanks again. The GPS module doesn't run at 3.3V. It has a minimum voltage of 3.6V. Iâll substitute a 5V regulator, and add a resistor for the LED.
Judging by the regulator you use to feed the stepper motor, it draws less than 500mA at 5V.
Judging by the regulator you use to feed the GPS module, it draws less than 500mA at 3.3V. The OLED displays won't draw much.
Using linear regulators you'll lose more power on heat dissipation than you actually use.
Conclusion: you'd better use the switching power supply on board of the Nano Every (assuming you DO use a Nano Every).
