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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. enter image description here

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?

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  • \$\begingroup\$ You didn't ask, but L1 and L7 don't do much useful in this circuit. \$\endgroup\$
    – Spehro Pefhany
    Mar 22, 2016 at 22:31

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The battery of a car may be nominally 12V but the system produces lots of spike (transients) voltages far bigger than 12V.

The SM6T39A (39V) is a transient suppressor and limits the potential damage this spike could do.

The BAT diodes have one end tied to 3V3 and act as a clamp circuit. Any voltage in excess of about 3V5 (3V3 + forward diode drop) will be 'clamped' at this level so further protecting the (sensitive) inputs to the microcontroller.

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