Common ground for two power supplies

I have a project to control something (basically toggling a relay occasionally) with an MSP430 Launchpad. However, to make things just a wee complicated, I want to monitor the thing with my PC while I calibrate the settings (potentiometers to set duty cycle parameters).

Now the Launchpad gets its power from the USB while the PC gets its data fed from the USB. The relay takes its 5V from the (nominal) 12V that the relay is controlling (7805 regulator). A pair of transistors connect the MCU's output port to the relay. In order to get everything kosher, I should connect the two grounds - the Launchpad's 3.3V and the external circuit's 5/12 V (a switching power brick that actually puts out about 14.5V).

Problem: I set up the two sources without connecting the two and measured the voltage difference between the grounds: 0.03V was the highest with 0.01V being typical. Is this safe to connect?

If I wasn't going to monitor the thing with USB, it would be simple - I will ultimately just power the Launchpad from the 5V and no connection to the PC will be required - no more dual power source.

1. The USB grounds on the PC are the same.

Check the launch pad jumper options for powering the launchpad externally. You can buy USB isolators. I do have one. Reguires a separate 5V supply. About $40 (overseas from the US) , if I remember.

A conventional power brick's 'ground' would normally be floating, as would the PC (and hence USB) 'ground', i.e. neither is tied to an actual earth point. So if that is the case then IMHO connecting the grounds directly together should be OK.

Alec