I am trying to make a launch controller, which will turn on with a key. Like the one in this photo below:
closed as off-topic by jsotola, sempaiscuba, VE7JRO♦, Greenonline, Juraj Oct 24 '18 at 8:12
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Check for example here for a key switch (see picture below).
As you can see it has two pins. If the pins are connected the switch is closed (same like a normal switch). So you can just connect one wire to VCC / 5V and the other via a high value resistor to a GPIO pin and check for LOW or HIGH.
(note: in practice it's possible to use almost any (electric/mechanical) component as long as the number of pins is not too high to be reasonably used by an Arduino).
A key switch is just a normal switch. What you do with it depends on how you wire it and code for it.