Opto Isolated Switch Inputs

An excellent input protection can be achieved by opto isolated inputs. Note that there is no electrical connection between the control panel switch and the input of the IC buffer. A closed switch connecting ground to the cathode of the LED completes the circuit consisting of the light emitting diode in the optocoupler and the resistor R1. The light emitted by the LED turns "on" the phototransistor which connects ground to the input of the IC buffer.

Opto isolated  inputs are used in many arcade games e.g. Daytona, Sega Rally.