Expression pedal

[night-B]

I would like to replace a pot by an expression pedal. I dont know if i must choose a phasel or a midi pedal, and how to connect it. Can somebody help me with this project?

[jkokura]

I might be able to, but I'll need more details.

What pot on what project? Do you have an exp pedal already?

Jacob

[night-B]

I want to put the pedal instead of pot 1 = pulse pot of the douleflush tremolo.
I don't have a pedal yet so I'm wondering wich type (midi or phasel) would be the easyest to adapt to the schem... Thank you in advance

[jkokura]

I'm not sure what you mean by Midi or Phasel. I'm pretty sure neither of those are really what you need though. You'd probably simply need a TRS expression pedal like the M-Audio type, or the Ernie Ball VPJR with a TRS to dual TS cable... You can google how to use a VPJR as an expression pedal for more details.

I think I'll try and make a new post on how to install an Expression pedal output for any typical pot...

Jacob

[night-B]

Thanks I will explore the vpjr way 

[jkokura]

k, so long story short - I wrote out a HUGE post with lots of info in it. Spent a half hour on it, lots of links, and before I was ready to post, my wife needed on the puter. So I opened a window for her, and went to the Post Office and back. Came back, and my wife's sister had logged us out on the computer, meaning I lost all my work.

So here's try number two, which will likely take me less time but be better overall.

So there's good news! This is entirely possible to do. Essentially, I'll walk you through how Expression pedals work with DIY pedals, and how you can replace just about any pot function with an Expression pedal.

For more information about Pots in general, you should go here: http://www.beavisaudio.com/techpages/Pots/

Here's a basic rundown. Pots are essentially variable resistors. Lug 3 is often used as the input of the resistor, so the electrical signal goes in there, and then out through lug number one. In between the two is a resistive track (wafer) that the signal will travel through. Lug 2 of any pot is usually called a wiper, and this is what makes a pot variable - the wiper moves along the wafer allowing you to vary the resistance! This is particularly helpful for us to know, because not all pots connect to the circuit the same.

Some pots are used as variable resistors, and some pots are used as voltage dividers. I don't even understand all that, but what you need to know right now is that they're different, and they allow a pot to operate the same way but do different functions. The variable resistor one is the easier to explain and understand.

If you open up the double flush documents in another tab or window, it will be helpful: http://www.madbeanpedals.com/projects/DoubleFlush/docs/DoubleFlush_ver.1.pdf

If you take a look at the pot connections on the board, you'll see they're all labeled with 3 on the left, and 1 on the right. What's more, there are three different methods for connecting the pots seen. Take a look at where each lug connects to the other parts. On the volume pot, lug 3 is the input from the output cap (C5) and lug two connects to the output jack, while lug 1 is connected to ground. On the Intensity and Pulse pots, lug three is the input as well, but lugs 1 and 2 are connected together as well as to other circuit parts. On the Balance pot, lug 3 is the output to the LDR, Lug 1 is the output to one of the gain stages of the IC, and lug 2 is the input. The balance pot is a bit of a special case, and i'll leave that alone today.

However, the speed, intensity and volume pots all have something in common - lug 3 is the input, lug 2 is the output, and lug 1 doesn't seem to matter much. This is the variable resistance use of the pot as an example in a real life situation - the Double Flush tremolo.

first, we need to understand how an expression pedal works. There are two different kinds - voltage dividing and variable resistance. Remember that we figured out that the Pulse pot is variable resistance? That means we need a variable resistance expression pedal! This is good news and bad news - it's good news because at least we know this now, but it's bad news because variable resistance expression pedals are reliant on a specific pot value. If your original control is 50k, but the expression pedal has a 10k pot inside it will behave very differently when you use the expression pedal. For example, on the Double Flush, the intensity pot is 10kB and the pulse pot is 50kB. That would mean we'd need two different pots inside the expression pedal to accurately work each control. This is why the voltage dividing expression is more popular and used by many more companies. So, the easiest solution is to simply build your own expression pedal. This eliminates the need to make sure your pedal works with anyone's expression pedal because you're in control of both.

But our original poster wants to make the speed or 'pulse' function controllable with his foot rather than using a knob alone, so aside from the difficulty of finding the right expression pedal, There are two ways to accomplish this. Both methods are possible, the first being easier but less useful, the second being harder but allows for a more versatile pedal.

1) Replace the pulse pot entirely.  This means that there would be no speed control on the pedal itself and you would NEED to have an expression pedal to control it.

The parts needed for this are quite simple. We simply need a stereo insulated jack like this: http://www.smallbearelec.com/Detail.bok?no=771. What you would do to wire this up is send the connection for the pulse pot lug 3 to the ring, and the connection for the pulse pot 2 to the tip of the jack, and then lug 1 would go to the sleeve. When you plug your TRS cable into this jack, you'll connect down the cable to the expression pedal.

At the expression pedal, you'll need a second one of those jacks and a 50kB pot that's been properly installed into the treadle of the pedal (think empty wah). You'll need to connect the right lugs to the right locations again - connect lug 3 of the pot to the ring, lug 2 to the tip, and lug 1 to the sleeve. Ultimately, you could connect any lug of the pot to any lug of the jack as long as it's the same at both ends. That's simple, but the pedal won't work without the expression pedal, and don't forget that the cable needs to be a TRS or stereo one.

2) Add an expression out that would bypass the pot. This means you would have a knob to control speed normally, except when you plug an expression pedal in. When an expression pedal is plugged in, the knob on the pedal becomes useless.

This method is a little bit more challenging. It involves using a switching jack instead of a typical isolated stereo jack. Here's an example of one: http://www.smallbearelec.com/Detail.bok?no=1041 This is a two way switching jack (tip and sleeve switch, common ground). This jack will work without being isolated but ONLY in this particular case. In the case of the balance pot as an example, this jack and method would not work. The only reason we can get away with using this particular jack instead of a true three way switching jack is because this pot only NEEDS to have two lugs attached to the PCB for it to work, lug 1 simply connects to lug 2 at the pots.

Switching jacks work like this. When nothing is plugged into the jack, contact is made between the switching lugs and the regular lugs. When a jack is installed, the contacts against the plug are pushed away and lose contact with the switching lugs. Take a close look and see if you can see what I mean with this jack here: http://www.smallbearelec.com/Detail.bok?no=88 This is a one way switching jack (tip, common ground). See the little tab there against the plug contact? That's the switching mechanism. The tab marked "NC" is where you'd connect to that little switch. I use those particular jacks to create loops inside pedals that don't need a stomp switch to activate them.

Anyway, how you'd connect in this case is simple: From the PCB, connect the spot for lug 3 to the ring tab, connect the spot for lug 3 to the tip tab. Then attach the pot in the pedal to the switch lugs, lug 3 to the ring switch tab, lug 2 to the tip switch tab, and lug 1 to lug 2. Again, head down a TRS cable and at the pot in your wah enclosure attache the of the pot (lug 2) to the tip of your stereo jack, lug 3 to the ring, and lug 1 to the sleeve. At the wah, you don't need a switching jack, a simple stereo jack will do the job, but try and isolate it so you can use it with multiple pedals if you can.

So there you go. That's the two ways you could do it in this particular instance. Note these two things:

You will most likely have to build your own expression pedal for this to work with your pedal. There is no guarantee a commercially available expression pedal will work for you. Find a cheap old wah and gut it, or a even a used expression pedal and open it up - it may just need the right pot value.

This is not a solution for all pots. this only works with these kinds of variable resistor type pots. For example, the volume pot in this build would need lug one of the pot in the pedal and the expression pedal to connect to ground, which might require a bit different wiring. Also, the balance pot would be quite different and might need you to use different jacks and wiring. I'm not sure where to get a 3 way switching jack (tip, sleeve and ground switches) but I'm sure if I wanted to I could get a hold of one. Not to mention the need to worry about isolation with some expression jacks and not others...

Needless to say, it requires thinking out.

Boy, that's a big post. Lots of work, and I'm not even sure it's all correct. Feel free to say if you spot errors - they could just be typos, or be errors in my information. I'm not a know it all, I'm a know it some.

Jacob

[night-B]

Wow thank you so much for such detailed informations.
This is going to be very usefull!
Ill try to achieve my goal and this is going to be a lot of work...
Will post my building report ASAP.
Thanks again.
NightB

[nzCdog]

...Came back, and my wife's sister had logged us out on the computer, meaning I lost all my work.

GRRRRRR!   Thanks for the 2nd attempt, its a good read