I need to design a circuit for my vehicle tracking application and i am using pic micro-controller . I need to capture the iginition status of a vehicle using the micro-controller ,where i have searched the google and i have designed the voltage divider circuit.
this circuit is designed for 12v supply . I can understand that by using voltage divider is used in the circuit. i can check the pin as high or low which indicates(on or off).But some trucks have 24v battery in that case when i can change the resistance value. But i don't understand that how both the diodebat54c and SM6T39A protects the circuit and prevent my micro controller io pins? I have read the datasheet i dont get any clear idea. please explain me?