Best way to protect a digital output? I am looking for a way to protect my Arduino's digital outputs from unwanted, outside voltages. My project uses about 20 different outputs, each connected to a wire (run) that can range from 20 to 100 feet (I used cat5 cable, with each inner wire (randomly chosen) dedicated to a single digital in, out, +5, +9 or ground.)  There is a good change that outside voltages could impact these digital outs (or any other pin or power wire.)

I have a good resource on protecting digital in's but I ma not sure how to protect the digital outs.  Each needs to be able to both pull a connect component to ground, and push it up to 5 volts (am I saying this correctly?)

This should be able to protect the digital outs from shorts or 12v spikes.

How can I build this circuit?

Edit: it is difficult for me to describe this problem without mentioning the danger to all components. 

Note that ways of protecting digital inputs is HERE.
Some good results you can get with optocouplers.


It's electrically disconnects one part of your circuit from another.

For example, here is my IoT-ish device, which is detects water leaks in my bathroom:


I use the transformer (T1), which have two separate output windings, so, that windings does not connected electrically, only magnetically. The block in border "Sensing unit" doesn't have any wires with other scheme - only magnetic field, which is gives it power, and infrared light in the optocoupler (u2, PC817), which informs ESP01 about leak presence.

How does it work? Very simple! When the water shorts JP1 (long naked wires on the floor), Q1 opens and lights-up the IR diode in PC817. Light from this diode opens another transistor - and pulls the RxD PIN of ESP01 down. 

Small sketch, written on Arduino for ESP8266 periodically sends pin state by HTTPS on my web server, where monitor software, Nagios, checks the information age and the pin state, and send to me notifications, when something goes wrong.