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How to implement a common ground

I have successfully transfered my SpeakJet module off the breadboard and onto some vero stripboard. I could power the SpeakJet module and the PicAxe module from one shared battery pack, but I'm wondering if it is possible to power each module separately with battery packs at different voltages (nominally, 4.5v for PicAxe and 6v for SpeakJet). The PicAxe communicates with the SpeakJet over a single wire. Do I simply need to connect the ground (0v) of each module together, or is there more to it than that?

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Thanks folks.

What I'd finding amazing, is that if I power the PicAxe and connect it to the SpeakJet, there's enough power going over the data wire to activate the SpeakJet module without a battery at all. In this config they seem to get out of sync pretty quickly so it deteriorates. It will be interesting to see if this persists once I put a common ground in.

Got it in one!
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