Voltage Controlled Variable Resistor

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Wien Bridge Oscillator Tutorial and Theory - Basic Electronics Tutorials

A well known voltage controlled variable resistor is the fet. Its Vgs-Rds relation is not linear but this can be compensated for. Another option is using a photoresistor in place of the variable resistor and controll it by light. V-R is not linear also, response is slower and not totally deterministic, (ther's some latency and mem effect involved) but it can be usefull in some apps. S.M.

Another way could be a servo controlled potentiometer. Has advantage of saving last setting from power down.

You could try search in GLOBALSPEC for "voltage controlled resistor". That's what it is for, it should identify manufacturers who have a ready engineered solution, with specification and description of operation.

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LED and LDR optic coupler.

Gajanan Phadte