I think we need some circuit diagrams . Depending on your circuit , disconnecting the ground may have put a reverse voltage or a higher voltage onto the Arduino than it can tolerate - disconnecting via the ground pin is never a good idea , and maybe the root cause .You need to be careful with your wiring and have an understanding of what you are doing with Connecting voltages from other devices operating above 5v. Higher than 5v voltages onto pins other than Vin can destroy it .... as you may have found .
If you're connecting and disconnecting power and or grounds when voltage is present, that would be why things are failing.
"Motor" and "smoke" sound like a missing (or misused) flyback diode.Another possibility is electrostatic discharge (from not grounding yourself when you touch the circuitry).
Hi,Have you got fuses on your power supply lines, especially if you are using batteries that are capable of 5A or more.Any short and the current will, will as you have found, make the smoke appear.How are you connecting your project components?Can you post a picture of your latest smoking controller?Thanks.. Tom..
That means i should have had a ON/OFF switch for the batteries?
Please provide circuit diagrams as requested in the first reply.