Replacing an expression pedal with an automated device

[Xanthene]

I need help in building a fairly simple circuit. I don't have anyone else around that could help me with this. I understand the basics of electronics and can put together circuit boards with the help of schematics. But when it comes time to create my own design, I have no idea where to start.

So can anyone help me with this project?

Here it goes.

I want to build a circuit that has one in and one out. I want this circuit to be an automatic variable resistor which acts as follows.

The Variable resistance will go from 1 K ohm (min) to 13K ohm (max). The resistance will be changed via a (1) sine wave, (2) square wave or (3) triangle wave oscillation. The user can determine the speed of change of the wave selected and also its amplitude. So for example, a sine wave can be selected and oscillate at 1hz(rate potentiometer) and the max amplitude of the change is set at 5K Ohm (amplitude potentiometer). The maximum resistance is 5K Ohm in this case.

There should be (3) knobs/pots: (1) wave selector, (1) rate, (1) amplitude.

Please, your help is appreciated. Compensation for your time will be given via paypal.

This device would replace an expression pedal (which is simply a potentiometer of 12-13Kohms). The idea is that it would automate the change instead of having to do it manually with a pedal and foot action.

The Rate function would go from 0.05Hz to 2Hz. The Amplitude function would allow the oscillation to change the resistance from 1K Ohm to 13K Ohm. So for example, if it were set at 1K Ohm, there would be no change in the resistance. Then Finally, the Wave selector would select what kind of form the change would be (Sine, Square or Sawtooth [slow rise to sharp fall and sharp rise to slow fall])

Hopefully this makes sense.

[expanoncolin]

I did this when I made the bent MM-4.  You are going to want to first build an oscillator, then feed the oscillator into an H11F3 or photocell to get the variable resistance.  Sine vs. Triangle modulation is basically the same 90% of the time, it is definitely not worth the extra work to get a sine wave.  My oscillator of choice is this one:

http://experimentalistsanonymous.com/di ... %20LFO.gif

You can get a different range out of the LFO by using a larger pot and changing that cap value.  You should feed the output of this into a simple, noninverting buffer:

http://www.seas.upenn.edu/~ese205/Labs0 ... age019.gif

You now have three op amps, might as well put it all in a quad op amp package.  Then, you might want to wire a simple volume pot (one side lug to the noninverting buffer output, one side lug to Vb, and the center lug is the out) into another noninverting buffer - this is the LFO amount pot.  Wire the output of this second noninvering buffer, whose output should go into a 470 ohm or so resistor, into the + input of the H11F3.  - input to ground, then you have 2 pins for the varying resistance.  If the expression pedal uses 3 connections, one for center, and two sides, then you will need to wire an inverting buffer, and a second H11F3.

-Colin

[Xanthene]

Thanks man! It actually is a 3 connection expression pedal, can you explain those detaisl further?

I really appreciate this.

[expanoncolin]

Thanks man! It actually is a 3 connection expression pedal, can you explain those detaisl further?

I really appreciate this.

OK, this makes things harder.  There is probably a way to do it with 4 op amps and just use a quad op amp chip, but the method that will definitely work is to send the output of the second noninverting buffer into a simple, unity gain inverting amplifier:

http://www.ozhiker.com/electronics/arch ... ic/inv.gif

R1 = R2 = 100k or 220k.  The + input should NOT be connected to ground in your case, it should be connected to Vb.  Then, take this output and send it through another 470 ohm or so resistor, into another H11F3's + input.  H11F3 - to ground... then you have two H11F3's whose resistances are varying but are opposite.  So connect together two of the H11F3 "resistance" pins, this is the "center" lug, and each remaining H11F3 "resistance" pin is a "side" lug of the pot.

[Xanthene]

Wow, this is a bit more complicated then I thought. Considering I'm not really very theoretical, would it be possible to draw the schematic of this device for me? This is a neat device that others would probably enjoy too as it allows you to be electronically expressive instead of manually.

[expanoncolin]

Wow, this is a bit more complicated then I thought. Considering I'm not really very theoretical, would it be possible to draw the schematic of this device for me? This is a neat device that others would probably enjoy too as it allows you to be electronically expressive instead of manually.  This is by no means a simple circuit to design or build.

-Colin

[Xanthene]

I'm just a more visual then litteral person. I'll be able to get it, I'll just have to play around with it. I'll let you know how it goes. I thank you for your time and help!

[Xanthene]

How much resistance will that give me anyways?

[expanoncolin]

How much resistance will that give me anyways?

The 470 ohm resistor is what scales the resistance.  Lower this resistance (NEVER LESS THAN 220 OHM!) for less, raise it for more.  I think 470 will give you a max of between 10-20k, you will have to play around with that value.

-Colin

[Xanthene]

For the Op I got a TL084. But I'm unsure as to what kind of capacitors to get. One of them on the drawing has no value. I'm trying to get the first section (oscillator) working first. I tried a few odd cap values but to no avail. Let me know.

I appreciate your help

[expanoncolin]

For the Op I got a TL084. But I'm unsure as to what kind of capacitors to get. One of them on the drawing has no value. I'm trying to get the first section (oscillator) working first. I tried a few odd cap values but to no avail. Let me know.

I appreciate your help

10 uF, 1 uF, 100 uF will all work there.

-Colin

[Xanthene]

I'm still having problems with it.

Has anybody else built this circuit before, if so, do you have the schematic?

I keep going through every step and still nothing. Could it be one of the values that's wrong?

[expanoncolin]

I'm still having problems with it.

Has anybody else built this circuit before, if so, do you have the schematic?

I keep going through every step and still nothing. Could it be one of the values that's wrong?

Is the lfo oscillating?  I have built the LFO circuit I posted hundreds of times.  What are your voltages on pins 3, 4, 6, and 8?

-Colin

[Xanthene]

3- 3.45v
4- 0v
6- 3.45v
8- 6.90v

My triangle wave isn't working, but the square is. How can I make the speed range greater? I want to have a good long range of sweep. As for the triangle wave, I think cap is polarized. How do I test that?

[expanoncolin]

3- 3.45v
4- 0v
6- 3.45v
8- 6.90v

My triangle wave isn't working, but the square is. How can I make the speed range greater? I want to have a good long range of sweep. As for the triangle wave, I think cap is polarized. How do I test that?

Are you powering this with 9V?  It should be 9V, I am not sure if the op amp you have works on just 7V like that.  The capacitor from pin 1 to 2 should be NON polarized, 1 uF, electrolytic or poly or whatever.  The other capacitor should be polarized.  A capacitor is polarized if it has which side is negative or positive on it.  You can extend the speed range by raising the value of the 100k pot... try perhaps making the 47k resistor 10k, and using a 1M audio/log taper pot.  Changing the 1 uF capacitor's value will change the overall range, but not make it bigger or smaller-smaller cap values means higher frequency, bigger cap values means lower frequency.

-Colin