Activating a circuit with a change in resistance
Arduino can do this (microcontroller board). You will want to make a voltage divider, where one resistor is fixed and another one is the one which changes resistance. The output of this voltage divider will therefore change as the resistance changes. Arduino can poll that value all the time and notify or do whatever else (e.g. activate smth else) when you want.
Difficulty: minimal
Required skills: most basic arduino skills, namely GPIO, ADC, nothing more.
This one you can figure out in a pair of days.
You will need to watch like 5 Arduino videos for that, the very intro, nothing more really. Unless you need to activate smth complex there, but it's probably nothing one can't do with an arduino.