I am trying to make a circuit that will can "auto-sense" the resistance of another circuit's resister, and allow me cut the resister out and automatically click over the substitution resistor. I am somewhat new to designing circuits, but am able to read and build them. I was thinking a mosfet would be great for the clicking over with a gate, but the resistance sensing and tying it together is what is getting me. I know there are circuits that compare voltage, but is it even possible to do with resistance?

 

is it even possible to do with resistance?

Not, in general, when it's in the circuit.
The other parts of the circuit connected to it can distort the real value of the resistance.

 

It sounds like an interesting idea. It can be done over a limited voltage, current, and frequency range.A MOSFET can work in some cases. Another way is to vary the feed back in an opamp circuit.


In this circuit R2 can be simulated by a duty cycle modulated (like PWM) switch followed by a low pass filter. That would provide accurate programmable resistance across the input terminals.