> I was wondering if it were possible to use a mosfet/bjt triggered
> by 16f628?

An electronic variable resistance can be made from FETs. For
example a 4066 (which has FET gates) or similar quad CMOS
switch can be controlled by PWM to get an effective resistance
between Ron and Roff. Note though that Ron for a 4066 is a few
tens of ohms. The best in a chip is possibly the ADG736 and
that's 4R5. Discrete FETs can have Ron in milliohms

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