This article explains the method in which you can safeguard your Arduino from dangerous spikes from reaching the I/O ports. The article describes two easy ways to accomplish the required safety precautions…
It is very common to use a Arduino to measure an analog voltage or interface a switch to the digital input pins for various projects. When this is done in an controlled environment it is perfectly normal but when we have to receive such voltages from an external source and when we have a switch placed very far from the Arduino in a industrial place we have to be ready to expect unfavorable voltages which might enter the Arduino that might just “fry” the board.
Its ideal to have the input voltages being measured with-in the range of supplied voltage to the Arduino.
The resistor R1 which is a low value one is to limit the incoming current. This resistor ensures our Arduino does not get damaged due to high current flow. The capacitor C1 is used to suppress and ground the transients caused by static discharges in the line.
The second stage is made of two signal diodes. The combination of these diodes are called as “clamping” or “catcher” diodes.
The diode D1 will start to conduct when the input voltage goes over the supply voltage, there by suppressing the high voltage from reaching the Arduino board. The diode D2 will conduct the negative spikes in the similar way.