Pedal-O-Matic foot pedal module

this looks awesome . would love something like this set up ! . are you going to make PCB’s for this ?. I have not the skill or time to strip board or design something of this complexity . I am sure there would be many people interested in purchasing my self included .

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Hmm, make a pcb… Well, the schematic is the first step in a KiCad design. As the pictures show I already have a strip board version. I do need to check whether everything works the way I want it though if 4 pedals are connected to it (still waiting for some 6 mm jack outlets). I will have a look at what it takes to design a PCB tomorrow. I’m not promising anything here.

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no pressure , you are just farther in on something I have dreamt of . I am sure if you put the time into making it work , I / we could make it worth your while .

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I like your idea of -5v to +5v foot pedal ,it was discussed on another thread and dismissed , but it would be great to go up octaves and down too with your foot . I could build the pedal to do this , its just the electronics part is beyond my skill level at this point .

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The -5V to 5V range is added to accommodate Euro rack modules and in my ecosystem a few Behringer synths use that. What is the CV range for the Kosmo modules?

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I don’t think that’s any more standardized for Kosmo than for Eurorack, but @antoine.pasde2 has collected some information here:

but I think the first question should be “which CV” – anything with a summing node and either an existing on-board offset (e.g. coarse tune, octave selector, etc) or one external CV plugged in can handle an additional negative CV just fine. Other circuits might be less willing to deal with negative CVs.

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Exactly that brought me to the 2 ranges. Some of the semi modular Behringer synths I have ( Model D, Neutron, Craft ) allow a -5V to 5V CV signal to certain inputs, but only 0V - 5V to others. I’m not sure though whether the latter are protected against negative voltages ( a diode would be adequate ). I suppose I could maybe measure the presence of a diode on the input when the device is not switched on …

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I updated the schematic.

I split the arduino and voltage reference bit from the op-amp circuits and put them on separate sheets. The main sheet looks like this:

The sub sheets for the 4 pedals are basically the same. The one for pedal 01 looks like this:

[Edit] Unfortunately there are a few mistakes in this schematic. The + and - of U1A have been swapped and the resistor on pin 4 of the midi connector should be connected to +5V (not +12V).

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Since the Arduino already reads the pedals’ analog values, you could use it to drive the yellow and green LEDs too. You could then get rid of two opamps, a bunch of resistors and diodes and maybe even the transistors.

You could also use the second switch of the DPDT to change the feedback resistor of the first opamp U1A to select a gain of 1 or 2, and get rid of U1B. (BTW you’ve got the feedback to the + input of U1B so that needs to be fixed anyways.)

The MIDI out pull-up should be to +5V, not +12V.

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Thank you for looking at the schematic so thoroughly!

[Edit] The LEDs are not essential and could have been left out. That will save 2 opamps, the transistors and a few diodes. Then the TL074 could be replaced with one TL072 per foot pedal.

Indeed I could have the arduino control all the leds. I started out with analog electronics because I needed to do all calculations in that domain to make the 2 voltage ranges possible and I added the LEDs afterwards but didn’t think of the arduino controlling the LEDs via PWM, so I implemented that in analog electronics as well. Had I gone ‘your way’, that would have saved a few components indeed! But your suggestion would not work out of the box. For this to work the arduino should also be able to read negative voltages from the foot pedals. So those would then need to be connected between +12V and -12V. And then also the configuration of the controlling op-amps UxA and UxB who are responsible for creating the 2 voltages ranges needs to be adapted.

The feedback for U1B is to the negative input, to make it a 2x amplifier. But it is a non inverting amplifier, so that may seem a bit odd at first. But it is drawn correctly. As you can see all op-amp circuits are using in a non-inverting way.

[quote=“antoine.pasde2, post:15, topic:1210”]
The MIDI out pull-up should be to +5V, not +12V.
[/quote] Yep, that is not what it should be, should be a +5V label.

I will change the schematics to make the corrections needed and leave the rest as is because I already have a working prototype. [Edit] I think that I will draw a sketch of the circuit which includes controlling the LEDs via the arduino and changing the foot-pedal circuitry. This sketch will however remain untested until I need a second Pedal-O-Matic. I leave simplifying the design to anyone who wants to. Feel free to contact me if you need any help.

Thanks again for looking at the schematic!

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Sorry, I meant the feedback on U1A is on the + input.

That is not a feedback loop in the sense of a normal feedback loop. The circuit subtracts the voltage on the switch from the voltage VP_x.

Agreed.
You could have the Arduino read the voltage at pin 2 of SW1A to determine if the range is bipolar or not, and control the LEDs accordingly.

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If you don’t have negative feedback, the opamp becomes a comparator and you’ll only get two discreet output values.

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Here’s a differential amplifier topology:
image
When R1 = R2 and R3 = R4, Vout = (R3/R1)(V2-V1).

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[EDIT] Rats! The + and - input of the U2A op-amp are swapped in the drawing. As you can see this is not the case in the hand drawn original design / schematic, I posted earlier. I will change this.

And then one would need 8 pwm outputs of the Nano to be able to light the LEDs proportionally to the voltage and 4 ‘plain’ ports for the midi LEDs. Or use less ports in combination with a multiplexing network. There are various possible approaches to this.

U1B has feedback on the inverting input, which is as it should be, right?

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Yep, U1B is drawn correctly. I will change the schematic now and correct the text in my previous post.

In the original sub schematic unfortunately the + and - inputs of the (subtracting) op-amp U2A were swapped. This has been corrected in this schematic:


If you are not interested in using the LEDs to show the foot pedal position, you can leave U2C and U2D out and all signal circuitry connected to them. You could then use a TL072 in stead and adapt the schematic to the pin out of that op-amp.

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