Author Topic: DIY digital pot  (Read 9563 times)

potul

DIY digital pot
« on: May 15, 2007, 11:31:54 AM »
Hi guys,

I was googling the web for information on this but I couldn't find. Not sure if it's something already used or not.
The idea came to my mind when thinking on a "programable" analog pedal, where you could store presets.
Would it be feasible to build a pwm controlled pot, using a CD4053 and a couple of resistors? My idea would be to put 2 resitors in series, with the cd4053 shorting one of them at a time. The joint between the 2 resistors would be the central lug of the pot and the outer points lug 1 and 3. The pot value (max R) would be the same as one of the resistors, and the minimum 0 (as usual).

Does it seem feasible? Or maybe it would be better to use optocouplers instead?

Potul

A.S.P.

Re: DIY digital pot
« Reply #1 on: May 15, 2007, 12:33:24 PM »
« Last Edit: May 15, 2007, 12:36:48 PM by A.S.P. »
Analogue Signal Processing

Chuck

Re: DIY digital pot
« Reply #2 on: May 15, 2007, 03:39:10 PM »
I've used the I2C controlled digital pot with good success.
Controlled by a PIC.

potul

Re: DIY digital pot
« Reply #3 on: May 15, 2007, 04:29:39 PM »
http://www.diystompboxes.com/smfforum/index.php?topic=42789.0


Thanks for the link, that's an interesting topic I didn't find...
Anyway, my idea was to try to get out of the digital pots. I'm not really wanting to build anything, I was just wondering if something similar to the "PWM trem" from Goefex http://geofex.com/Article_Folders/ASMOP/asmoptrem.gif could be implemented in another way to get a "pwm pot". He's basically putting a switch in series with a fixed resistor and opening/closing at high freq the switch to get an apparent resistance bigger than the resistor.
What will happen if instead of putting a switch in series to the resistor we put it in parallel. Will we get an apparent resistance smaller than the fixed resistor (between 0 and R depending on the duty cycle)? If this is possible, then with an electronically controlled dpdt (like the cd4053) we could control a pair of resistors and make them behave like a pot.

Another different story is if it could be used in any audio circuit without introducing noise. Probably it could be used to control LFOs or similar things...

Could it work?

Potul


MetalGuy

Re: DIY digital pot
« Reply #4 on: May 15, 2007, 04:43:30 PM »
Excuse me but some reason the idea of motorized pots sounds to me like going back to 5.25" floppy disk drives. Especially when /as noted/ the supply is limited, not to mention the very limited choice of pot values.

Chuck

Re: DIY digital pot
« Reply #5 on: May 16, 2007, 02:36:00 PM »
Whats wrong with 5.25 inch disk drives?   :)

A.S.P.

Re: DIY digital pot
« Reply #6 on: May 16, 2007, 02:55:51 PM »
before that, there was tape;)
Analogue Signal Processing

MetalGuy

Re: DIY digital pot
« Reply #7 on: May 16, 2007, 04:23:28 PM »
Quote
Whats wrong with 5.25 inch disk drives?

Nothing wrong. Just limited supply, big, noisy etc.

Auke Haarsma

Re: DIY digital pot
« Reply #8 on: May 18, 2007, 04:13:31 AM »
Quote
Whats wrong with 5.25 inch disk drives?

Nothing wrong. Just limited supply, big, noisy etc.


Sounds like a major MOJO thing to me...
NOS Ge trannies....limited supply, big, noisy ;) and very very very MOJO loaded ;)

MetalGuy

Re: DIY digital pot
« Reply #9 on: May 18, 2007, 04:31:29 PM »
OK. Can you point to a source for 220k, 500k and 1M motorized pots?


Chuck

Re: DIY digital pot
« Reply #10 on: May 18, 2007, 07:14:41 PM »
I believe Alps makes motorized pots.
Check Mouser and Digikey.

David

Re: DIY digital pot
« Reply #11 on: May 21, 2007, 03:54:32 PM »
Why do they have to be motorized?

Check out the "Rock 'n' Control" article on "The tone god's" site.

MetalGuy

Re: DIY digital pot
« Reply #12 on: May 21, 2007, 04:09:03 PM »
Quote
Why do they have to be motorized?

They don't have to be motorized at all. Thanks but no 5.25" floppy drives for me.

David

Re: DIY digital pot
« Reply #13 on: May 21, 2007, 05:54:21 PM »
Quote
Why do they have to be motorized?

They don't have to be motorized at all. Thanks but no 5.25" floppy drives for me.

What we have here is a failure to communicate.  Check out this thread:  http://www.diystompboxes.com/smfforum/index.php?topic=44586.msg324509#msg324509
Start at reply #5.

MetalGuy

Re: DIY digital pot
« Reply #14 on: May 22, 2007, 04:42:38 PM »
I'm in favor of VACTROLS as well. There's a long thread about the LDRs as pots:

http://music-electronics-forum.com/showthread.php?t=1015

I've done some experiments with my own DIY 4 pin dual cell LDRs /that's what MESA uses/ and the results are promising. You need two such 4 pin LDRs in order to imitate a real pot /with taper/. Control of these pot is achieved via regular pots /or rotary encoders/ -> ADC ->uCU -> digital pots etc. so you can build and memorize presets.

David

Re: DIY digital pot
« Reply #15 on: July 09, 2007, 02:37:23 PM »
Hey, Potul and Metal Guy:

Are you making any progress with this?  It turns out that I need something similar so I've started researching it.  I don't want to steal your thunder, though.

MetalGuy

Re: DIY digital pot
« Reply #16 on: July 09, 2007, 05:01:14 PM »
Actually all necessary control modules are built and ready but there are still some software tweaks to be made. In the next several weeks I'm planning some experiments  on one of my tube amps to see /to hear in fact/ how it works.
In the meantime /if you haven't noticed/ H&K released fully programmable all tube amps with FX module. I'm curious how they did it - with optocouplers ot with digital pots.
« Last Edit: July 09, 2007, 05:03:37 PM by MetalGuy »