The firmware for the electronic stamp is downloadable here: inker-1.0.zip.
A movie of me using an earlier prototype (using another kind of cartridge) is viewable here.

The pinout of the cartridge seems to be mis-printed in the magazine (amongst others, R3 is missing) so here is the correct pinout of the model 40 HP-cartridge:
Pinout cartridge. Seen when looked at the back of the cartridge, with the printing head pointing downwards.

Gnd	Gnd		Gnd	Gnd
C1	R		R	C
R4	Gnd		Gnd	R1
C2	R		R	C
R0	Gnd		Gnd	R5
C3	R		R	C
R3	Gnd		Gnd	R2
C4	R6		R7	C

R0-7 en C1-4 are used in the schematic, the rest isn't.

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© Jeroen Domburg