I'm just looking to build a GPIO protection circuit for my two in-use inputs as detailed here:
(using a zener and resistor)
Is it as simple as it appears to put a red LED between the zener and GND, to create a "error light" ?
The zener will permit max 3V3 (in this case), with a LED in series the limit voltage will increase, the LED will start to turn on at ~0V7 and go as high as 1V4 ~ 2V0 in some cases (meaning you have upto 5V0 going to the GPIO). If the input is really badly connected then the LED will blow and you then have no protection at all (in this circuit it would only take 12V)
Note also that the original circuit offers no protection against negative voltages.
By far the best way to protect your RPi is to use a driver or buffer component that is more rugged and more expendable. You can buy darlington driver ICs with 8 chanels on for 60pence from CPC http://cpc.farnell.com/jsp/search/produ ... tt=SC08607
. This will also give a simple level conversion as well as driving moderate loads (each output can sink 500mA with package total of 2A5)