An 8 in 16 out  interface for the Raspberry PI model B+

The issue 2 board was rather awkward if you wanted to use 24 GPIO ports. The model B+ makes things very much easier. There now is only a single connector, enabling a much neater design.

My finished interface is on the left. I did not bother drawing a circuit diagram, so I have marked all the resistor values. The three chips are all ULN 2803 Darlington drivers. As the orientation notch is not well visible:  It faces North with all three chips. Three 100nF capacitors are also used.

If you use any of my BBC BASIC programs, be aware that all the input/output assignments have to be altered - else you might get some strange results.

On 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 this design.

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:

0V,22,27,4,17,10,9,14,15

Output is on the right and goes from bottom to top:

11,5,6,13,19,26,21,20,16,12,7,8,25,24,23,18,0V,Vref

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.

Link to the Bplus interface drawfile

Back to the interface section  

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