I'm also on my way to give this a shot. Can't promise anything though…
A few considerations and ideas:
- 4½ digits is what I consider the minimum and yet maximum reasonable for an open source multimeter. Everything else requires a lot more effort in the analog frontend.
- System performance could be optimized with software tricks by using the internal temperature sensor (this is just an idea, I have done absolutely no research on this so far)
- Use 4 full-fledged, fully configurable channels - switching between V and A could be done with bipolar relais (this is also just an idea, and no further research has gone into that)
Notes on the design draft, which I have done too early due to a cold and boredom in bed:
- The draft below uses components which you can buy, including the case
- The case is absolutely huge with 209x98x34mm (bigger than the Metrahit 25S), and a bumper is also available
- The 4mm plugs fit in the case with a headroom of ~3mm
- Uses tact switches and the MEC NaviMec solution. The navigation serves dual purpose - when a menu is selected, it serves as navigation buttons. Outside of menus it selects the multimeter functions. Pressing 2 buttons activates the function in that segment (no idea how to describe that)
- It's using the 2.7" Sharp Memory LCD, which is slightly too small for the case, but probably I can fit everything in the Bopla BS 703 case, which is 20mm smaller in height than the BS 803.
- It's notable that the cost for buttons, display and casing is already 138.79€ including VAT at quantities of 25 copies. The price includes the switches, LCD, case and input jacks (64€ for the input switches alone). Let's be realistic about the price - even if this project works out, it won't come cheap, unless it's possible to save costs somewhere. However, I'm not intending to trade of awesomeness-factor against cheaper parts; personally, even if I didn't do the project, I would be absolutely willing to pay 300-400€ for such a meter, as it replaces up to 4 meters.
That draft was put together in probably 3 hours. Things might be missing, plain wrong, not doable etc - yes, I will figure that out during the further design process.