Two different voltage input to be monitored by microcontroller
Just add a resistive divider to your ignition input, to make the output around 4-5V. 
For the output of your OR gate, add a resistive divider as well.

A very simple OR port can also be made using two diodes, as shown below:







simulate this circuit &ndash; Schematic created using CircuitLab

Note that R1 and R3 form a resistive divider, to make the 5V into less than 2V. The 3 resistors form a divider for the 16V, making it less than 2V as well.