problem with ground connection...

hello friends, I have made a microcontroller based circuit that needs two supplies. one with +5V to drive MCU(and some other components) and +12V to drive uln2803. I don't know where I am making mistake, but when I connect two voltages to the circuit from two different supply boards I get two different(and both are unsatisfying :sad results depending upon from which power supply board I take the ground wire. i.e taking ground from +5V supply board, I get :
1)+5V pot. difference between 5V pin and GND pin on my MCU board. but...
2) 0V pot. difference between 12V pin and that GND pin and also between 12V pin and 5V pin :shock: and
3)+12V pot. difference between that 12V pin and the GND pin on power supply board from which this +12V has been taken.

on the other hand, when I connect ground wire to the GND pin on +12V power supply board I get different results:
1)+12V pot. difference between the 12V pin and GND pin on my MCU board.
2)+11.30V pot. difference between the 12V pin and 5V pin
3)0V pot. difference between +5V pin and GND pin on MCU board(which is now taken from 12V supply board).:shock:
4)5V pot. difference between +5V pin and GND on 5V supply board.

I have no idea about these results. could someone please tell me what mistake I have done, or where I am lagging. thank you very much .

post the circuit

You need to connect the ground from both supplies.

Grounding can a somewhat complex issue, due to voltage drop, current loops and noise pick-up.

Using two independent supplies is not a good idea, as you have found out.

Both supplies must be connected to the common ground for the circuit, because each voltage must be relative to the same ground potential. But...

If both supplies are connected to the mains Earth, then you will be making a loop with the Earth/ground connections. That is why two separate power supplies is a bad idea. If only one (or neither) of them is connected to Earth ('floating') then the situation might be simpler, but still not good because the separate supplies might be floating at different potentials relative to Earth and each other.

Also, if both supplies are Earthed and you connect only one supply ground to the circuit, then the circuit might appear to work because you are still connecting both supply grounds because the grounds are connected together at Earth. That might cause issues though because the ground connection to the non-ground-connected supply is a long way, through the Earth wiring, and probably has voltage drop and noise pickup.

In short: You will have to connect both supply grounds to your circuit ground together (at the same point preferably), but it's not ideal. Make sure both supplies are plugged into the same mains circuit, preferably into a double socket, to minimise the potential loop.