Right. I've finished Uni for the year, and although I do have a summer job I've got some time to actually put quite a bit of effort into this.
I've been looking at circuit diagrams, methods of checking battery level and current metering, and I think I've realised a few things.
Firstly, replacing the main ULA with a PIC is completely possible and shouldn't be too hard. I _might_ have a working prototype by the end of the week.
Secondly, it may be easier to make a whole new POD, Sinclair didn't use any Analogue to Digital Converters, so some of the inputs to the ULA aren't in ADC friendly form, I'll have to check that though. Hopefully I should be getting an old oscilloscope from our electronics lab today (if the lab techs are nice!) so I'll be able to properly analyse exactly what's going on in checking current & voltage. (Voltage is easy, current should be but the inputs to the ULA regarding current are confusing me)
The only difficulties I can see are, as I've said, getting the signals from the control box into a friendly form for analysis. Once that's done everything should be easy. Controlling the LEDs and buzzer, for example, is really easy!
Also, would anyone be interested in some sort of data logging? There's not that much information available, but I could get it to record current & voltage over time, for download to computer for analysis. Might be a little more work but shouldn't be too hard to do... That wouldn't be in the first version though!