I have a circuit which controls the volume on a speaker by a wheel which is attached to a variable resistor - I want to reproduce this but instead of using a manually operated variable resistor, I want to use ...something else instead - Ideally something where you can apply a voltage to change the resistance from low to high.
I've done a little research but I think I'm being stumped by not knowing what I'm actually looking for.
Answer
You can use a transistor to do this. Although less common that the other types, a JFET works much like a voltage controlled variable resistance. You'd have to apply an analog voltage to the gate to get a specific resistance. You'd have to be careful about the range of this voltage. The Drain and Source would act as the effective two terminal resistor. Even a mosfet has a linear resistive region so this is not your only option. There are many other options as well which I'm sure will be mentioned.
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