interphase is a open-source mechanical keyboard. Kudos to reversebias, as I got inspired by his fascinating mitosis keyboard (https://github.com/reversebias/mitosis-hardware). The only problem was the lack of keys for a full german layout + number row.
Therefor, interphase is a kind of ISO-compatible layout with some minor modifications.
Update on battery life: The AAA batteries (Duracell) per keyboard-half are still going strong. After 5 months of heavy usage at work, they only dropped to ~1.4V. Therefor the total lifetime of the batteries will be approximateley 1 - 1.5 years, as the voltage regulator will work down to 0.35V.
As identified in Durburz/interphase-firmware#6 it is strictly necessary to use the C variant of the MCP1640