Expression Pedal /Control Voltage input for SPIN FV-1

[knutolai]

Hi

I want to improve the Expression input on a FV-1 project I'm working on. This is my current solution:


This circuit is obviously very simplified, but it illustrates my implementation. The circuit consists of a switched TRS socket with 4 pins in use. It expects a passive expression pedal input like the one illustrated on the left. A 100R resistor (R1) serves as a primitive  protection against connection of a mono jack that would otherwise short 3.3v to ground. When connected correctly however R1 and the expression pedal forms a voltage divider with a range from (nearly) 3.3v to ground. This signal is fed via a CD4053 into the SPIN FV-1, both running on 3.3V. The CD4053 is used to toggle between the different control sources; a knob or the expression pedal input. The Sleeve Switch pin is used to detect that an expression pedal is connected (not important for this thread).

The circuit obviously has some drawbacks:
-No over-voltage protection on the TIP pin (can kill the CD4053 and/or SPIN FV-1)
-No negative-voltage protection in the TIP pin (can kill the CD4053 and/or SPIN FV-1)
-Cant be used with CV input where a mono jack is used
-Gives an incomplete range. 3.3v can't be reached because of R1

I want to amend these shortcomings with the following circuit, though I'm uncertain if I'm thinking straight:


What everything does:
-RV2 is a calibration trimpot for the expression input. Connect your expression pedal and adjust until you have 3.3v when in the max position (toe down/heel up).
-R1 serves as a protection resistor when a mono jack is connected. Also RV2 + R1 = 0 Ohm isn't useful for expression calibration. Note that R1 is connected to 9V in this setting (not 3.3V)
-The schottky diode is meant to serve as a negative voltage protection (has a low forward voltage)
-R2 and the zener diode is meant to serve as over-voltage protection

As stated I'm a little uncertain whether this will work or if I've included some unnecessary components. I have a feeling R3 and R4 may be unnecessary. I'm very much looking for input here!

Thanks!

Below are circuits for when an Expression pedal or a Control Voltage (CV) is connected:

[Firesledge]

If the 9V supply is not regulated, you won't get reliable results from the RV2 trimpot. You could use a regulator to get 3.3V on the ring pin. The TL431 is a voltage reference that can also absorb current which is good if someone plugs an audio output jack. Manual calibration is trivial. Put the protective resistor on its supply side.

For D2, you could use a schottky diode with the +3.3V on the cathode. The forward voltage is low enough not to fry the ICs and will ensure that you can actually reach 100% of the reference voltage. R3 and R4 may actually be harmful: current flowing through them will cause a voltage drop, which will cancel the protection provided by the diode. The 1N5817 supports 1A as forward current which should be enough.

[knutolai]

If the 9V supply is not regulated, you won't get reliable results from the RV2 trimpot. You could use a regulator to get 3.3V on the ring pin. The TL431 is a voltage reference that can also absorb current which is good if someone plugs an audio output jack. Manual calibration is trivial. Put the protective resistor on its supply side.

http://www.taydaelectronics.com/datasheets/A-140.pdf


Do I read the datasheet correctly that I don't need to supply the TL431 from a regulator? It will supply a steady voltage with different power supplies? I take it that in the picture above "R" takes the place of the "protective resistor".

For D2, you could use a schottky diode with the +3.3V on the cathode. The forward voltage is low enough not to fry the ICs and will ensure that you can actually reach 100% of the reference voltage. R3 and R4 may actually be harmful: current flowing through them will cause a voltage drop, which will cancel the protection provided by the diode. The 1N5817 supports 1A as forward current which should be enough.

Thanks! New proposed circuit: (Vo = 3.3V)

[ElectricDruid]

Why not use a stereo jack with N/C contacts? Like that, you could wire the pot to the N/C contacts and when the pedal was inserted, it'd take over from the pot. Saves the 4053, and some of the problems associated with it. Unless you're doing more clever stuff depending on which one is plugged in?

Tom

[knutolai]

Why not use a stereo jack with N/C contacts?

Great suggestion, but I want to be able to map the expression/CV signal to different pot inputs.

I've ordered some TL431 chips to test Firesledges suggestion. Will report my findings here

[ElectricDruid]

Errmm...do all the pots like that?!?

I suppose you *could*. You'd have a row of expression jacks on the back of the pedal, and by plugging into any which one of them, you could override that pot with the pedal. Ok, maybe you don't need 'em all, but you could include two or three.

It's a basic and space-in-enclosure-eating way of achieving what you're aiming to do a way what is more cleverer .

Do let us know what you come up with. I'm intrigued!

T.

[knutolai]

Errmm...do all the pots like that?!?

That was my initial idea too.  It's gonna end up in a vertical 1590BB with top mounted jacks. So space is very much an issue here. Would be so great if everything could be housed in some enormous 1590F with every conceivable function without the pedalboard getting crowded.

[knutolai]

At long last I managed to finish this circuit. There's a lot of other circuit involved that does not serve any purpose in regard to the subject of the thread so I've left a lot out of the schematic below.



Whats going on:
1. Expression control for P0, P1 & P2. The microcontroller (MCU) detects when the Ring Switch on the Expression Jack is disconnected from the ring. Controlsignals from the MCU maps the expression/CV input to one of the pots (P0, P1 ..). T = tip, R = Ring, Rsw = Ring Switch, S = Sleeve of the jack socket. I ended up using a trimpot for adjusting the voltage of the TL431 regulator. Works fine, easy to tune with a 1turn trimpot. R32 and the 1N5817 (5817) diodes are for overvoltage and negative voltage protection. I haven't been able to test the circuit with a lot of CV-outputting devices, but so far everything seems to work fine. SW1 does the mapping.

2. In addition to the expression/CV control I also added a footswitch to swap between two physical pots for 1 parameter. A preset of sorts labeled as "FUNC". All three pots voltages travel via the EXP and FUNC functions before arriving at the SPIN FV-1 (the pads P0, P1 and P2). SW2 does the mapping while SW3 turns the function on and off. FUNC1 is the preset pot.

3. SW4 is the bypass switch for the pedal. There's a standard TQ2-L2-5V relay connected to the MCU that I cut away for this post.

Thank you for helping out! So great to be able to have CV and EXP on the same jack