A bit of background. I want to make a pcb the interfaces with a wiegand proximity reader that is 12v. So I plan to put a 12v switching PSU on the board which will supply the pi with 5v and also give me the 12v I need to power the reader. The reader interface will look like this.
1 - 12v
2 - 0v
3 - Data input with voltage divider (data is 5v)
4 - Data input with voltage divider (data is 5v)
5 - LED Output (using transistor to ground type switch)
6 - Beeper Output (using transistor to ground type switch)
Now 1 and 2 are obviously just power. But 3-6 are indirectly attached to the GPIOs. Now I want to protect the Pi of course but my primary concern is to protect/disable the reader pins in the event of a short which will most likely be if the cable to the reader is comprimised. So it is likely to be a 12v short to 2,3,4,5 or 6. I will limit the output to about 250mAh as most readers are around 150 - 200.
How can I protect this input from shorts tampering without compromising the overall PSU? Would a polyfuse on every input/output be sufficient?