I will look into that. In principle this should not pose much problem. It will require additional set of MOSFETs and optocouplers. Opamp that I chose is already a dual channel one, so no problem there.
I think my project is quite economical:
T245 handpiece - 63€ (TME)
some tip, eg C245939 - 35€ (TME)
24V, 150VA toroidal transformer - 19€ (toroidy.pl - local Polish manufacturer), almost any other transformer can be used (i use 12V/300VA in the prototype and it works just fine)
STM32F051K6 - 2.5€ (Farnell)
RPC1-6RB-12P connector - 2.8€ (Farnell)
PCB - ~2€/piece from Elecrow if it fits on 10x10cm format
MCP6V02 opamp - 2.2€
Enclosure -
http://www.kradex.com.pl/shop?product=287 Local Polish manufacturer, they have english version of the website but i dunno if they ship abroad. If anything, I can buy and distribute that to people. Anyway - appropriate enclosure is under 2€.
Other bits and pieces like displays, encoder, optocouplers, trimpots and the like can be had from anywhere or from junk, so I won;t bother listing them, as anybody probably has them in their junkpile. So the control units sums up to 30€ for major components. Even assuming that the rest will cost another 30€, this is still cheap
