Uncommon grounding issues

Derp

Hello.
I am having a little issue.

I am currently using the BOE revision D with a BS2 while designing and testing components of a circuit for a project.
I am triggering a 2n3904 to switch an LED on and off.

The LED is powered by 12V on a separate powered breadboard.
The LED has roughly 30 mA flowing through it, which makes me a little apprehensive about tying it to Vss on the BOE

The issue is this:
The ground voltage level of the Powered Board is roughly 5V higher than the ground voltage level of the BOE (Vss)
I have tried adjusting the -V source to use as a ground reference instead, but all that seems to do is increase the ground reference in reference to the BOE if that makes any sense.

Are my fears unfounded in tying all the grounds to the BOE's Vss or will that really burn out that pin at 30 mA even though the 12V (30 mA) source is originated from the powered bb?

Ack, that image is tiny, sorry.

Phil Pilgrim (PhiPi)

It all depends on why the ground levels are different. If it's a low-impedance difference, yes, connecting them together could result in excessive ground currents and could damage components. How did you measure the potential difference? With a high-impedance multimeter? What happens if you connect a 1K resistor between the two grounds? Do you still measure 5V? If the resistor bleeds off any capacitive charge that's the cause of the difference, connecting the two together should not be a problem.

If this really is an issue, though, you can always resort to optical isolation to drive the LED.

-Phil

Derp

I'll try that thanks. It shouldn't be an issue in final assembly as both circuits will be mounted to the same board.