the right is the pcb layout of the interface. The circuit uses all
available GPIO lines except GPIO 2 and GPIO 3. Those two are awkward,
because they have pull-up resistors, which makes them unsuitable for
The circuit performed well and I have had
no bother with it so far. I would be most interested to hear from anyone who had problems with this design.
The input lines are on the left of the board and are connected - from bottom to top - to GPIO pins:
Output is on the right and goes from bottom to top:
Where Vref is the voltage driving the circuit connected to the
outputs. The ULN2803 needs to know this voltage in order to function. It
can be up to 50V.
Finally here is the usual link for RISC OS users who might prefer to use a drawfile to make their own pcb.