RPM Gauge Arduino.... Need the explanation of the Zener diode, what is the fuction on this circuit
The Darlington will have two diode drops from the NANO output logic level and thus the Vmax is reduced by 1.2 to 1.4V but it has a very low output impedance.

The Zener normally has no effect except when inductive loads are switched off , then the diode side conducts and clamps the out to 12+0.7V and if the power is turned OFF when the output is high, it will result in a negative kickback clamped by the Zener side conducting at 12-Vz. to prevent negative pulses. It could have been done with 2 diodes instead, one to each rail.