Author Topic: digital pot as variable resistor  (Read 10118 times)

Dimitree

digital pot as variable resistor
« on: February 17, 2014, 10:32:32 AM »
hi everyone,
I connected an arduino and a MCP41010 digital pot (10K), wired as variable resistor, into my Line6 M13 expression pedal input. The M13 expression input works perfect with my 25K ernie ball expression pedal.
But using the digital pot, I can't get the full range..it does not go to 100%.
The resistance of the digital pot that I measure is 0.5ohm at minimum digital value and 9.6K at maximum value.
I tried with 50K and 100K digital pots too, it's the same. And unfortunately I can't calibrate it inside the M13.
Why it work like that? can I fix it someway?

Danila-master

Re: digital pot as variable resistor
« Reply #1 on: February 17, 2014, 11:35:08 AM »
Minimal resistance of these pots is about 50-100 OHms -  it is ok.

You can scale values for 50K potentiometer to obtain 0..25K range.

Could you please share your schematic and code here? 

Dimitree

Re: digital pot as variable resistor
« Reply #2 on: February 17, 2014, 12:17:57 PM »
no I cannot scale it, since it looks like the M13 is expecting the expression pedal to be a voltage divider, indeed there's voltage over the expression input jack.
So the R value of the digital pot is not important.

About schematic/code, I simply followed those instructions:
http://www.instructables.com/id/Digital-Potentiometer-MCP41100-and-Arduino/step3/Arduino-Code/

indeed it works fine if I read the resistance with a multimeter, but for some reason it doesn't work well as voltage divider.
I believe I should add some resistor somewhere?

Dimitree

Re: digital pot as variable resistor
« Reply #3 on: February 17, 2014, 12:27:17 PM »
another option is that the M13 is expecting 0 ohm or close. The digital pot have a minimum resistance and that could be the reason why the M13 won't go to it's minimum setting. I checked with DMM and indeed my Ernie Ball expression pedal goes all the way down to 0 ohm.

mth5044

Re: digital pot as variable resistor
« Reply #4 on: February 17, 2014, 01:11:19 PM »
Why don't you check the voltage rather than the resistance? Compare the ernie ball to the digital pot (full on and off), that might prove your hypothesis.

Danila-master

Re: digital pot as variable resistor
« Reply #5 on: February 17, 2014, 01:35:29 PM »
Totally agree.
This digital pot can be used as a voltage divider
You need to check pinout of M13 jack to see if you have there any positive voltage...
If M13 is being controlled by voltage - it is great, you can try adding a negative offset to your digital pot to obtain 0V value.
Anyway try to use a pair of real resistors as a potentiometer 100 Ohm + 50K and connect them to the  M13 and you will see if the minimal resistance is a critical issue or not.

Sorry for my English, I'm Russian. :)

Dimitree

Re: digital pot as variable resistor
« Reply #6 on: February 17, 2014, 02:57:29 PM »
thanks
Voltage with the expression pedal is 0V to 2.2V, while voltage with no expression pedal connected is 3.2V.
The cable needed for the expression input is a simple mono cable, so only 2 terminals

mth5044

Re: digital pot as variable resistor
« Reply #7 on: February 17, 2014, 03:17:55 PM »
Are your measurments with the ernie ball or the digital pot?

The 3.2V, that is what the M13 is putting out, right? I'd imagine it would want something similar back, so it looks like the 2.2V you are giving back isn't enough.

Are you sure the M13 uses a mono cable? I would imagine you need a stereo cable for the power sent from the unit, the return voltage and ground.

Dimitree

Re: digital pot as variable resistor
« Reply #8 on: February 17, 2014, 03:32:01 PM »
the measurements are with the ernie ball connected, 2.2V is the max voltage, 0V the minimum. Yes the M13 is putting out 3.2V thru a mono cable..but when I connect the ernie ball there's that 1V drop.

Danila-master

Re: digital pot as variable resistor
« Reply #9 on: February 17, 2014, 03:38:32 PM »
You can try to connect a 50 or 100 K regular potentiometer and to see what's happening.
Did you try to use Ernie Ball with a stereo cable ?

Dimitree

Re: digital pot as variable resistor
« Reply #10 on: February 17, 2014, 03:46:09 PM »
Ok I can try that but why? I alrrady know that using a regular 10K pot using only two wires from my breadboard to m13 input works perfect. The input is mono so a stereo cable will be useless

mth5044

Re: digital pot as variable resistor
« Reply #11 on: February 17, 2014, 03:59:59 PM »
When you use the ernie ball, the jack is insulated right?

Where is the 1V drop coming from? If all that is in the Ernie Ball is a series resistor of 25k between the V send and V return of the M13 (which it has to be if you are using a mono cable  as you'd need stereo for a volage divider...?), at max you should be reading 3.2V out, then with the full 25k you'd get something like 0.5V. Does the manual state anything about the needed supporting circuitry for this?

Dimitree

Re: digital pot as variable resistor
« Reply #12 on: February 17, 2014, 04:20:29 PM »
If you read at the end of this article:

http://www.strymon.net/2010/07/12/strymon-tech-corner-1-anatomy-of-an-expression-pedal/

It says that:

The Line6 EX1 is the only one we’ve see that works differently, with only a simple resistor divider and a mono cable. The nice thing about their solution is that it uses a mono cable.

My ernie ball is a passive volume pedal, so maybe that's why is working well

Danila-master

Re: digital pot as variable resistor
« Reply #13 on: February 21, 2014, 05:45:26 PM »
Ok I can try that but why? I alrrady know that using a regular 10K pot using only two wires from my breadboard to m13 input works perfect. The input is mono so a stereo cable will be useless

Hello Friend.

I bought one digital pot and performed some tests.
Please try to connect arduino ground with m1's ground.

Please let me know the result.

Best regards,

Daniil

Dimitree

Re: digital pot as variable resistor
« Reply #14 on: February 25, 2014, 09:57:40 AM »
thanks for the help, I reconnected again the digital pot and now the minimum resistance doesn't seem to be an issue, but now the digital pot is floating when I put it at maximum value

mth5044

Re: digital pot as variable resistor
« Reply #15 on: February 25, 2014, 12:00:49 PM »
How have you change the setup and what do you mean by floating?

Danila-master

Re: digital pot as variable resistor
« Reply #16 on: February 25, 2014, 12:51:00 PM »
thanks for the help, I reconnected again the digital pot and now the minimum resistance doesn't seem to be an issue, but now the digital pot is floating when I put it at maximum value

I had the same issue with the PT2399 based delay.
There was some kind of modulated sounds and grounding solved this issue.

You can try to parse parameters from Arduino via serial port.

Mojsisije

Re: digital pot as variable resistor
« Reply #17 on: March 10, 2014, 01:16:54 PM »
Try connecting the digital pot in this way:
Say you have ends of the pot as pins A and B and the wiper as W, heel down being wiper at B and toe down wiper at A. Connect B to the sleeve of a TRS jack, and W to the tip, and then connect it with a mono cable to the M13.

Alternatively, and that's how I use it with an AD5206, B to sleeve, A to ring and W to tip, use a stereo cable with the Ring disconnected at the M13's end. I use it with an M9 so YMMV, but it's worth a try.

Dimitree

Re: digital pot as variable resistor
« Reply #18 on: March 10, 2014, 04:30:20 PM »
Hi guys today I tried with digital ground connected to m13 and it works perfect amd as intended. Wiper resistance is not at issue and there is no floating.
Thanks everyone

ElectricDruid

Re: digital pot as variable resistor
« Reply #19 on: July 12, 2014, 07:40:38 AM »
hi everyone,
I connected an arduino and a MCP41010 digital pot (10K), wired as variable resistor, into my Line6 M13 expression pedal input. The M13 expression input works perfect with my 25K ernie ball expression pedal.
But using the digital pot, I can't get the full range..it does not go to 100%.
The resistance of the digital pot that I measure is 0.5ohm at minimum digital value and 9.6K at maximum value.
I tried with 50K and 100K digital pots too, it's the same. And unfortunately I can't calibrate it inside the M13.
Why it work like that? can I fix it someway?

Sorry for the late post, but I just came across this, and something occurred to me. Seeing as you're trying to use this digipot to control an expression pedal input from an Arduino, why not just use the Arduino to generate the expression voltage directly?

I don't know but I'd guess the M113 is expecting 0-5V. You can check by seeing what it puts across your ernie ball pedal. You'll find X volts across the pot. Then you use the PWM output on the Arduino, followed by a LPF and possibly another op-amp to provide some gain if required. You could replace the digipot with a dual op-amp and you'd be done.

Just a thought.