Originally I made this question on the Electrical Engineering site but it was migrated to the Rasberry Pi site.
https://raspberrypi.stackexchange.com/q/36627/34975
It was later closed as off topic on the raspberry pi site - a decision I agree with as it the fact my scenario uses a Pi really has no bearing on the question posed.
So is the question I asked off topic for this site as well or was someone being a bit over zealous?