Programmable Volume Pedal Ideas

[LiquidMetal]

Hello!

First, I want to apologize for placing this post here...it is 50/50 analog/digital and I was not sure where to put it.

I'd like to build a volume pedal with a user-programmable swell curve. I want to use a rotary encoder to get the treadle position, a micro-controller (most likely AVR) and either a LCD/LDR or a digital potentiometer to control the signal. The AVR family was chosen because it offers an easy PWM implementation in case I go the LED/LDR road. The pedal should be able to use one of few pre-programmed  swell curves initially, the ultimate goal is to upload new curves via USB or MIDI.

I will appreciate if you can help me with few questions I have regarding this project:

* Is this a good approach in general
* Is there a better solution than using a rotary encoder to read the treadle position
* Can you suggest a particular rotary encoder type for this application
* Which is going to be better/easier to use - LED/LDR couple or a digital potentiometer
* Is it necessary to have a input/output buffer

Thanks!

[Taylor]

In general, I don't like to ask people "why?" when they have some specific idea of what they want. I own a bass guitar with 8 strings, quartertone frets, Hipshot detuners on all 8 strings, and an infinite sustainer pickup. I got really sick of people asking me why I would want such a thing. 

But I'll gently ask why you need to do that digitally. Microcontrollers are cool and can do lots of interesting things. In this case I wonder if you might not achieve a better result with analog. A couple of things come to mind:

-digital pots are usually run on 5v I think, so they clip in a gross way if anything bigger than that gets in there.
-LDRs have their own weird non-linear response. Either they will stomp all over your carefully crafted curves, or you will need to design your (lookup tables?) with a specific LDR in the circuit. Not just a particular type, but one particular unit.
-rotary encoders are not designed to be operated the way you work a volume pedal. They will probably degrade quickly as they won't be meant for the amount of use a volume pedal gets.

You can do an amazing amount of taper control using resistors. There is a great thread on this forum where somebody did all kinds of charts showing the exotic curves you can get just putting resistors across the pot terminals in different ways. I'll see if I can dig it up. R.G.'s "secret life of pots" on Geofex is a very good start in any case.

Apologies if you know all this already and are 3 steps ahead of my notes here - just putting some info out there.

[JKowalski]

-rotary encoders are not designed to be operated the way you work a volume pedal. They will probably degrade quickly as they won't be meant for the amount of use a volume pedal gets.

You are thinking of the wrong rotary encoders. They are often used in industrial equipment or less impressive but still extremely rugged instrumentation/equipment as position sensors. I guarantee you can find a sturdy one.

Or you could just use a potentiometer and select a uC with a ADC converter. Either way would work fine...

As for the volume adjustment, maybe consider an OTA? Would be predictable, and offer smooth adjustment.


IMO I don't think this pedal would be very useful. Do you really need to have this much control?

I would try to make a pedal with some switches to change the response of the treadle passively to 4 or so types of tapers and try it out, and then see if I was satisfied or not.

[jasperoosthoek]

-digital pots are usually run on 5v I think, so they clip in a gross way if anything bigger than that gets in there.

Hmm, 5 volts peak to peak is quite loud for a guitar/stompbox signal. The AD7376 can handle +/-15 volts or 28 volts and is even available in a PDIP package! If you are able to handle SMD components your choices are increased a lot: http://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/products/index.html Many can handle +/-5 volts, which ideal for stompboxes. Analog seems to have a lot more DIP packages then Maxim.

[joegagan]

i like where you are going with this. i had a similar thought a while back, but am now in a more primitive mode again.



View My Video

the little green leds were too far away, i reworked it with red ones up close to the photocells. also, in the picture above, the shutter shapes had not been carved into the disc yet.

need to get back to this experiment, so far do not have workable readings.

[~arph]

Keep in mind that the AVR's normal PWM frequency runs in the audible range. I believe you can boost it beyond 20kHz though.

[earthtonesaudio]

I have a suggestion even simpler than analog:
Take a look at Morley's LED/slit/LDR design.  Simply stick multiple copies under one treadle, each with a different slit shape, and you have your presets.  You could even do something like A/B/Both and add LDRs in parallel to get 3 presets from two LED/slit/LDR modules.

[slacker]

I own a bass guitar with 8 strings, quartertone frets, Hipshot detuners on all 8 strings, and an infinite sustainer pickup. I got really sick of people asking me why I would want such a thing. 

Apologies for the potential thread hijack, I haven't got anything useful to add, but I demand pictures of this beast 

[joegagan]

i like morley's approach. not quite as fond of the cardboard used for the shutter tho. i've seen warpage issues.

[DougH]

I have an Ernie Ball volume pedal with an extra pot and a knob for adjusting the volume pedal pot taper. It's very effective. Am I missing something?!?

[pjwhite]

This sounds like a neat idea. 
A comment for DougH:  BY going digital, in addition to subtle tweaks in the response taper, you could also do things like reversing the response entirely or having a low-high-low curve.

For the implementation, I would use a pot and ADC for the input, as jkowalski suggested, instead of the encoder, and calibrate it by taking discrete measurements at every few degrees of revolution (or pedal angle).
To control the level of the audio signal digitally, I would use a multiplying DAC -- feed the level control into the DAC digital inputs and feed the audio signal to the DAC reference input. 

[merlinb]

A further option (similar to the Morley) is to use a magnet on the treadle and a Hall-effect sensor to convert treadle position into an analogue control voltage, that could be a sampled by an ADC.

[potul]

Let me add 2 comments:

The good think about going digital is that once you have the basic hardware implemented, it's a simple matter of programming to add some interesting functionalities to your pedal:

-Tremolo, with whatever LFO curve you want to have.
-Sequenced sound chopping.
-With a little more effort (monitoring in signal), you could alse create something like a SlowGear
-In theory, apply any response curve you desire
-etc..
And, it's fun to explore the digital DIY.

But if you simply want a volume pedal, I'll stick to analog.

I would go into using a pot instead of an encoder. I don't see the point of using an encoder for this application, when you are interested in the absolute position of the pedal, not the relative movement of it.
If you go into the encoder way, you will need to recalibrate the encoder position every time you power up the unit, otherwise it will not know where the pedal is....

Regards,

Potul