12V signal convert to 5V without interfere
Use a couple of voltage comparators as shown below and select them for the speed you need.

R1 and R2 are used in conjunction with R5 and R6 to generate the hysteresis which will insure clean transitions at the comparator outputs, R3 and R4 are used to generate a 6 volt reference (the input switching point) for U1A and U1B, and R7 and R8 are pullups to 5V for the open collector outputs of the comparators.