Using single 12v DC power source for project

[PerryBebbington]

When you interrupt the current to an inductor, which is what a maglock is, the inductor tries to maintain the current. If the terminals of the inductor are open circuit the inductor has to produce a high voltage to have any chance of maintaining the current, possibly several hundred or 1000V. You don't want 1000V near a microcontroller.
See:

Flyback diodes and why you need them

Any component that relies on an electromagnet or coil of wire for its operation will be inductive, that is the nature of coils of wire. Common examples are relays, solenoid and most types of motor. It is standard practice to put a 'flyback' diode across any inductive load in order to catch the back emf from the inductor when the current is interrupted. This requirement is usually taken at face value without being backed up by any kind of test results or evidence. This tutorial provides the evid…

For what to do about it.