Circuit reads one resistance & alters another

[stonerbox]

Looking for help/ideas/a start on a circuit that would read resistance to ground at A (e.g Gtr volume pot) and in turn alter resistance to ground at B (e.g Emitter to ground). When resistance is low at A, resistance at B is supposed to go up.

Been thinking of using LDRs but all ideas are welcome.

[Matthew Sanford]

What will change the resistance? If LDRs you could set an LFO to one LED and it's inverted compliment to the other

[stonerbox]

That could absolutely work with a LDR or a FET. Great idea for part B.

I am however completely stuck and out of knowledge on how to do part A, reading resistance of guitars volume pot to ground and then altering the LFO.

[fryingpan]

The easiest thing to imagine for me is: oscillator outputting an ultrasonic frequency into a reference resistor, then into the pot. A circuit sensing the resulting AC voltage (voltmeter) at the end of the resistor, and outputting a current proportional to the voltage. Then , then profit.

[Matthew Sanford]

That sounds like a good way!

THis might sound silly, but could you just use a dual potentiometer so each turn keeps them the same? Or are you trying to make it warble this and that? I mean... what would be changing the resistance at point A where you couldn't just use that same function as dual for point B?

[idy]

Yes, dual pot, why not.

[stonerbox]

The easiest thing to imagine for me is: oscillator outputting an ultrasonic frequency into a reference resistor, then into the pot. A circuit sensing the resulting AC voltage (voltmeter) at the end of the resistor, and outputting a current proportional to the voltage. Then , then profit.

Sounds interesting but could you clarify a little bit more? I get the above hearing range oscillator. Then you want me to hook that up to a reference resistor. That reference resistor then goes to the the guitar's volume pot, or did I completely missunderstand you?

[stonerbox]

That sounds like a good way!

THis might sound silly, but could you just use a dual potentiometer so each turn keeps them the same? Or are you trying to make it warble this and that? I mean... what would be changing the resistance at point A where you couldn't just use that same function as dual for point B?

That would be my number one solution in cases like these but I need to alter the resistance at point B based on the guitar's volume pot resistance to ground.

[Matthew Sanford]

Ah, so not in the same circuit then, and my apologies for my ignorance, you've mentioned the guitar volume a couple of times. Maybe what you need is an envelope filter from the guitar's signal as it'd be relative to the position of it's pot to drive an LDR, so at least it would be relative though not exactly the same, and would need to work the inversion (bigger signal leaving larger resistance at point B).

[fryingpan]

The easiest thing to imagine for me is: oscillator outputting an ultrasonic frequency into a reference resistor, then into the pot. A circuit sensing the resulting AC voltage (voltmeter) at the end of the resistor, and outputting a current proportional to the voltage. Then , then profit.

Sounds interesting but could you clarify a little bit more? I get the above hearing range oscillator. Then you want me to hook that up to a reference resistor. That reference resistor then goes to the the guitar's volume pot, or did I completely missunderstand you?

I was just brainstorming. That said, I imagine a box that actually is the first pedal in the chain that you connect your guitar to. This pedal would act both as a buffer and actually sending this ultrasonic signal up to the guitar through a large resistor. For instance, 2M2. The guitar would see this resistor, into the opamp's output, and the buffer's input with its biasing resistor (10M for instance). So from the reference resistor's point of view, the voltage drop of that ultrasonic signal would almost solely be determined by the volume pot's position.

But now that I think about it, you'd get some weird sensing, because at max volume, the pot is in parallel with the pickup (much lower value), and would only start working once you start working the pot. Or maybe not. A pickup has a lot of series inductance which opposes high frequencies, but it does have a parallel capacitance. Also, the tone pot, although maxed, will be a parallel impedance to the volume pot (but since you almost never use the tone pot on electric guitar, it may be irrelevant, it's just one more fixed value). You could try to simulate it, with the usual guitar pickup values (look them up) and replicate pickup going into the pot, then going into the tone pot, going into a large resistor, and connect this large resistor to an AC voltage generator set to something like 100mV @ 80kHz. If, by a quick search, you can model a guitar pickup with a 120p parallel capacitance to ground, then it won't work.

[ElectricDruid]

I don't really see *how* you can do this if the guitar's volume pot is in use as a volume pot at the same time.

Any attempt to send signals into the pot to measure it's position is liable to mess with the guitar's output, either by screwing up the signal or by interacting with the pickups. While you *could* try something that's outside the audio band, it all sounds like trouble waiting to happen to me - ultrasonic whining, heterodyning, whatever. Maybe I'm being pessamistic and it could be made to work, but it's certainly not simple.

Attempts to measure the position using the output volume are doomed because (as we know) guitarists don't always play at the same level, so a given output level X might be "played normally, pot turned 75% up" or might be "played hard, turned only 50% up" or might be "absolutely hammered, and volume barely turned up at all". You can't tell without also knowing the input volume (which would be possible, I suppose). But what would you do if no notes are being played? Then you can't know the pot's position even if you detected the input level too.

Replacing the volume pot with a dual pot makes life *so* much easier. One half does the volume function. The other half acts as a voltage divider across some convenient voltage (9V battery?) and then the wiper provides an output control voltage which represents the pot position. That control voltage can be used to control...well, anything, pretty much. In the case of needing a controllable resistance to ground, you could go the basic way with a FET, or the more complicated way with an OTA (there's a voltage-controlled grounded R example in the full version of the LM13700 datasheet).

HTH

[fryingpan]

I don't really see *how* you can do this if the guitar's volume pot is in use as a volume pot at the same time.

Any attempt to send signals into the pot to measure it's position is liable to mess with the guitar's output, either by screwing up the signal or by interacting with the pickups. While you *could* try something that's outside the audio band, it all sounds like trouble waiting to happen to me - ultrasonic whining, heterodyning, whatever. Maybe I'm being pessamistic and it could be made to work, but it's certainly not simple.

Attempts to measure the position using the output volume are doomed because (as we know) guitarists don't always play at the same level, so a given output level X might be "played normally, pot turned 75% up" or might be "played hard, turned only 50% up" or might be "absolutely hammered, and volume barely turned up at all". You can't tell without also knowing the input volume (which would be possible, I suppose). But what would you do if no notes are being played? Then you can't know the pot's position even if you detected the input level too.

Replacing the volume pot with a dual pot makes life *so* much easier. One half does the volume function. The other half acts as a voltage divider across some convenient voltage (9V battery?) and then the wiper provides an output control voltage which represents the pot position. That control voltage can be used to control...well, anything, pretty much. In the case of needing a controllable resistance to ground, you could go the basic way with a FET, or the more complicated way with an OTA (there's a voltage-controlled grounded R example in the full version of the LM13700 datasheet).

HTH

Well, the tape bias signal is basically something very similar. It is filtered away at reproduction.

[ElectricDruid]

Well, the tape bias signal is basically something very similar. It is filtered away at reproduction.

Ok, so the plan is to feed in some ultrasonic signal, and then use the level of that to determine the pot position. Perhaps I'm more optimistic this morning, but that doesn't seem so bad. It's still not "simple" though. You need a buffer/mixer for the two signals to stop ultrasonics going to the pickups. You need an oscillator to produce the ultrasonics. You need a highpass filter and a level detector to get the pot position. And you need a lowpass filter to get your guitar signal back. The filters need to be pretty good because if there's any ultrasonics left in your guitar signal, it'll screw with your cheap digital pedals and play hell with your electric mistress. And if there's guitar signal in your ultrasonic signal, it'll mess with the level detection. That's quite a lot of stuff to build, especially if all we're doing is driving an LED and and LDR to get a resitance-to-ground.
So it's possible, but a dual pot is *way* simpler!

[fryingpan]

Well, the tape bias signal is basically something very similar. It is filtered away at reproduction.

Ok, so the plan is to feed in some ultrasonic signal, and then use the level of that to determine the pot position. Perhaps I'm more optimistic this morning, but that doesn't seem so bad. It's still not "simple" though. You need a buffer/mixer for the two signals to stop ultrasonics going to the pickups. You need an oscillator to produce the ultrasonics. You need a highpass filter and a level detector to get the pot position. And you need a lowpass filter to get your guitar signal back. The filters need to be pretty good because if there's any ultrasonics left in your guitar signal, it'll screw with your cheap digital pedals and play hell with your electric mistress. And if there's guitar signal in your ultrasonic signal, it'll mess with the level detection. That's quite a lot of stuff to build, especially if all we're doing is driving an LED and and LDR to get a resitance-to-ground.
So it's possible, but a dual pot is *way* simpler!

Yeah, of course, I was going for "no mods on the guitar" so that you can use whatever but this complicates things and you will still need to mod the guitar anyway.

[stonerbox]

I can understand why this particular circuit has not been done before.

[ElectricDruid]

Yeah, of course, I was going for "no mods on the guitar" so that you can use whatever but this complicates things and you will still need to mod the guitar anyway.

Stonerbox has an impressively complicated problem for something so apparently simple!! lol, often the way...

[amptramp]

Just a reminder that the word "transistor" was originally a contraction of "transfer resistor".  If you look at the tremolo section on the lower left of this Univox amp schematic, you will see a transistor used as a variable resistance driven by the LFO:

[Transistor-Transistor]

So OP what you want is the guitar volume to alter a resistance in a circuit?
I think what would be a lot easier is a level detection circuit with a vactrol(or just led/ldr) in parallel with the circuit you want to control. It would be very responsive to playing dynamics too but I think that would be a lot more doable than measuring the volume pot resistance.
 

[stonerbox]

Thank you all for your responses! In the end I solved the issue with the problematic stage. Let me explain...

I have a several-stage-fuzz on the breadboard. When biased at extremes and pushed hard it does some magical complex textures! But it would not clean up very well. Q3 was the problematic stage, it refused to clean up due to its output signal hitting the 0V floor and.. other things. If I biased it upwards, say with  a 2 Meg to 1/2V, it would clean up much better but the complex textures when cranked would become much too tame (have you guys figured out what type of fuzz it is yet?). I even tried a 10 Meg from 1/2V to base but it would still ruin the fuzz's complexity. Out of luck, patience and knowledge I said **** it and stuck some diodes in there and voilà it worked!
1/2V -> 10 Meg -> (A) 1N914 (C) -> (A) 1N5918 (C) -> Base of Q3. With this chain, it cleans up when guitar is low and retains it's extremes when the guitar is at full.