Looking for schematic to mimic an "auto sweeping" expression pedal?

[Yonatan]

I have a Boss FT-2 Dynamic Filter, which has a manual mode that allows you to control the cutoff of the filter with an expression pedal.  What I would like to do is built a circuit that plugs into the expression pedal input and automatically sweeps between a set range e.g. it would be equivalent to manually turning the cutoff freq knob back and forth at a fixed rate.

1. It should have controls for low (e.g. lowest cutoff freq), high (highest cutoff freq), and rate (how fast to sweep between the low and high).  Type of wave might be a plus e.g. triangle vs. sine.
2. I would also like to be able to control this rate with an expression pedal (meaning an expression pedal would plug into this circuit, which would plug into the FT-2's expression pedal input)

Can anyone point me to some circuit that accomplishes this?

To accomplish #1, I understand from this thread that the Boss/Roland expression pedal basically acting as a variable resistor, so it seems like I need a circuit that acts as a automatically changing variable resistor (OTA controlled by LFO?), the result of which gets put in between the tip/ring of the expression pedal input?
http://www.diystompboxes.com/smfforum/index.php?topic=34201.0
And then to accomplish #2 would be a matter of controlling the LFO's rate with an expression pedal instead of a rate pot?

[nocentelli]

You need to look at the FT-2 schematic to see how the exp input connects. I used a roland EV-5 with my oldf FT-2, and the EV-5 has a 10k linear pot: The FT2 might use all three connections, i.e. Tip, ring AND sleeve, and you would in that case need to replicate a voltage divider not a simple variable resistor.

[Yonatan]

I don't have the schematic for the FT-2, but the instruction manual calls for an EV-5, and after searching around, it seems like the EV-5 does indeed work as a voltage divider. 

So, any schematics for making a voltage divider that is driven by an LFO?

I see that geofx shows different ways that to get both higher AND lower resistance from an increase in control voltage, but I don't know how to turn that into something useable.
http://www.geofex.com/Article_Folders/LFOs/psuedorandom.htm#Driving%20Effects%20Circuits%20with%20the%20LFO%20Waveforms

[WaveshapeIllusions]

The LFO is the easy part. Getting it to vary resistance is the difficult part. I'm assuming it takes TRS. TS is easier, you'd just need one device. You could use two JFETs as variable resistors, each driven by half the waveform.

[ashcat_lt]

It is most likely that all you need is a control voltage that swings within the appropriate range to control the filter.  So build an LFO such that it swings in that appropriate range.  Then figure out which part of the TRS plug is supposed to carry the control voltage and run the output of the the LFO there.  Done.

To put it another way:  If it is in fact a TRS jack meant to connect to an expression pedal set up as a voltage divider, then you'll find that the T, R, and S attach to (say) lugs 1, 2, and 3 of the pot.  It's most likely that 1/T is some output voltage - the max desired control voltage, 3/S is ground (0V).  That leaves 2/R as the control voltage input.  Pretty sure the filter pedal won't give half a damn whether you're actually dividing the voltage it's supplying, or generating it some other way as long as it comes back on the ring, is referenced to the same ground point, and doesn't get too big.

[Yonatan]

Before building an LFO to test this idea, I'd like to test it by connecting a voltage to the control voltage and varying it (let's say with a pot for now).

One thing I'm not sure of:  What should I connect filter's output voltage (2/R) to?  Nothing, since I won't actually be using it?

And what control voltage is "too big" for the filter pedal to handle e.g. > 5v?

[ashcat_lt]

What I would do is stick a TRS cable into the jack while the pedal is powered up and running.  Then use a meter to measure voltage between each of the contacts and the case of the pedal.  This should tell you what Boss thought a good maximum voltage was, as well as help to sort out what goes where.

[Yonatan]

Update:

I'm breadboarding the ROG Tri-Vibe right now (http://www.runoffgroove.com/tri-vibe.html), and just finished the LFO section, so I thought I'd give this idea a try.

- The LFO of my Tri-Vibe seems to be putting out around 1.x to 4.x volts (It should be more really, but the battery I'm using is weak.  Anyway, if it gets too high, there is a depth knob to control it.)
- The voltage difference between T & S of the Boss FT-2 expression jack is about 5.6v (I used a mono cable, so I didn't measure the Ring)

So I thought that *if* this idea will work with just T & S, then then LFO from the Tri-Vibe would be a good candidate.  Well, after connecting the LFO to the tip, and ground from the LFO to the sleeve...

...It works!  Quite well.  Sounds almost like a phaser, but different.

So, this leaves the question of how to control the Speed knob of the LFO with an expression pedal.