Expression pedal idea...

[Drake120]

Well. Debugging a unit gives you a lot of time to think about other units :lol:

So. I want to build an expression pedal, as described for me here: http://www.diystompboxes.com/smfforum/index.php?topic=68017.0

But then a question appeared to me. How to solve the problem of different pot values in different units? I'm crazy about versatility, so of course my expression pedal will have to fit all of my other pedals :] So first, I came to an idea of mounting 6 pots in one wah enclosure. Well, possible, but hard to be done. Then I found that: http://diystompboxes.com/analogalchemy/emh/emh.html

So I thought about switching between different sets of resistors to mime different pot values. And then, boom! Why not mount these resistors on exp jack in each unit? Sleeve of jack will be lug 1, tip - lug 2, and ring - lug 3 (look at the first link). Yeah, so why not? :lol: It's to simple to work

I also have a question about the taper calculator - if I want log from linear, do I have to type 90%, or 10% there?

I know I want to much

(JD)^S

[F.V]

The problem as i see it is not that it isn't possible to do with resistors, it is, its just that no matter what you switch it to, the sweep of the pedal will only control a small range of the pot. For example, you want to use it with a pedal that needs a 10k resistor, the pot in the expression is 10k, no problem, but then you want to use it with a pedal which needs 100k, you add in resistors to the expression, and the overall resistance of the expression will be correct, but the pot will only control 10k worth of the pots sweep, only 10% of the knobs turn as it were.

[Faber]

But what if you were to use a 1M pot and then parallel resistors to get your desired value?  Sure some of the values wouldn't be exact, but they'd be close.

EDIT

For example, if you wanted a 100K pot, then you could parallel a 100K resistor with the 1M pot and it'd be pretty close to the same thing.  (In reality you'd want something closer to 111.5K, but 100K is close enough:  gives a max of 90K)  But then you'd have to get the pot down to 10% of its rotation left to half the resistance (assuming Linear Taper)...

But what if you used an audio taper?

This is about where it ends for me, can anyone with more knowledge help out?

[Drake120]

Yeah, I didn't mention it. I was planning to use 1M linear pot in the expression pedal, and then add some parallel resistors if I need some smaller pot. There's also a calculator for making logarithmic pot from linear pot in the link above

My question is, when I add these parallel resistors, would that 1M pot really become some smaller, desired value?

Thanks,
(JD)^S

[Faber]

Well, the maximum and minimum values would be close what you wanted, but if you used a linear pot, then the % travel of the pot wouldn't be equal to the % of the total resistance.

Let's just say you want a 100K pot, so you use a 1M pot and a 100K resistor.

1M pot (at 100%) and 100K resistor = ~90K
1M pot (at 75%) and 100K resistor = ~88K
1M pot (at 50%) and 100K resistor = ~83K
1M pot (at 25%) and 100K resistor = ~71K
1M pot (at 10%) and 100K resistor = 50K
1M pot (at 5%) and 100K resistor = ~33K
1M pot (at 0%) and 100K resistor = 0 ohms

What would have to happen is the 1M pot would have to be at only 10% of its resistance in the middle of its path or you get the values shown above which suggested to me that you'd want to use an audio pot instead of linear.

[birt]

if you use some setup like the "rock 'n control" with LDR's you use the pot of the expression pedal to control led brightness. any pot/voltage divider will get you about the same result.

[Faber]

Brilliant!!!

This is about where it ends for me, can anyone with more knowledge help out?

YAY!!!