Potentiometer Circuit Design

Question

For a fun project I've been trying to create some sort of circuit that behaves as follows:

• Two outputs, Vout1 and Vout2.
• One Input Vin
• 1 Controlling potentiometer Rpot from 0-MAX.
• Any combination (and amount) of BJTs, MOSFETs, capacitors, inductors, and resistors.

What I'm trying to accomplish is this: An AC signal enters the circuit. When Rpot is at some middle value say Rpot = .5MAX, both outputs are essentially in parallel and connected to the source Vin. Vout1 and Vout1 must receive the same signal (or very very close, 1-5mV off), and also must have at least 90% of the magnitude that Vin has (so if Vin has an amplitude of 1V, Vout1 and Vout2 must have an amplitude of ~.8V or better).

As Rpot is increased to its MAX value, I want to decrease the amount of the signal that Vout1 or Vout2 receives.

For Example: As Rpot increases to MAX, Vout1 = 1V amplitude while Vout2 = .5V And if Rpot approaches its 0 value then Vout2 = 1V amplitude while Vout1 = .5V

While the amplitudes I'm using are arbituary I think it gets what I'm trying to accomplish across. As I either increase or decrease the resistance of Rpot, I only want one output to attenuate at a time.

I've tried numerous voltage divider circuits and tried to use some BJTs to create some logical feedback but I'm running into wall. The closest I've gotten is using a voltage regulator that turns on when one channel is attenuating but I can't quite get it to work. Any help would be appreciated.

Thanks in advance!

Answer 1

Here's the simplest way to get what you asked for:-

^{simulate this circuit – Schematic created using CircuitLab}

Each side of VR1 forms a voltage divider in combination with R2/R3 or R1/R4. When the pot is centered each side is 5K so the signals at Vout1 and Vout2 are attenuated by less than 10%. Thus each output is more than 90% of Vin, and (assuming the pot can be precisely centered) exactly the same as the other.

As the pot is moved off center the resistance of one side decreases, causing more attenuation on that side. At full rotation that side is short, and then the series resistors divide the voltage by two. The other side now has less than 5% attenuation, ie. a slightly higher voltage than when the pot was centered, but that is still within your specification of being 90~100% of the input voltage.

If this was an audio circuit (eg. balance control) it would sound like one side stays at constant volume while the other side gets quieter. However the attenuation control is very non-linear, being 'soft' in the center and getting more aggressive towards either end.

Trying to make the attenuation control more linear is where you are likely to 'run into a wall'. You could use the pot to generate DC control voltages that can be shaped to suit whatever response you want, but then you need voltage controlled amplifiers or attenuators that can handle 1V signals with low distortion - which are not easy to make using only discrete components.

Answer 2

What you are describing is a kind of a weirdness that is not easily accomplished with linear parts- the pot is supposed to stop working for one signal when it passes the center of rotation and start affecting the other, and vice versa.

I suggest using a microcontroller with built-in ADC to digitize the pot value (preferably connect it as a voltage divider) and then controlling a dual digital pot with whatever function of pot angle you want to actually control your AC signal. You didn't mention the frequency, but this will work up to a few hundred kHz with little issue (of course the response to pot rotation will tend to be slower, but that probably doesn't matter).

If you use an MCU with a 10 bit ADC you'll have 512 steps on either side of center. This could be a very low parts-count and very simple circuit.

It would also be possible to do this with multipliers, op-amps connected as precision rectifiers and such like, but it would be quite expensive and complex and might require trimming to reach your desired performance.