grade my exp/cv input circuit for a fv-1 pedal to allow eurorack CV signals

[Boner]

Trying to make an exp/cv input for a fv-1 based pedal that will accept expression pedal inputs or a CV input from a eurorack. Negative voltages from the eurorack CV input get cut off so a user would have to fiddle with the CV value to make it positive only.


the op amp is to buffer the 3.3v supply to prevent short circuit the 3.3 supply when a TS jack is plugged in and the switch is to allow TS jacks from a eurorack setup by "removing" the ring and making it not short to the sleeve.

D3/D4 is for under-over voltage protection of the potentiometer input
Vcc is 9
Vin is potentiometer input

[antonis]

the op amp is to buffer the 3.3v supply to prevent short circuit the 3.3 supply when a TS jack is plugged in and the switch is to allow TS jacks from a eurorack setup by "removing" the ring and making it not short to the sleeve.

I'm not sure I got what you mean but op-amp buffer, as it is, can't buffer anything..
Either from input (voltage mode) or output (current  mode) standpoint..
(any input's ripple/fluctuation passes to output and any output short leads op-amp either to current-limit or oscillation state..)

[Boner]

(any input's ripple/fluctuation passes to output and any output short leads op-amp either to current-limit or oscillation state..)

well that doesnt sound good....

maybe buffer is the wrong word. The 3.3v supplies power to the FV-1 and the op-amp is to create some kind of separation/buffer/barrier so if a TS jack is plugged in, the 3.3v itself wont be shorted to ground so the fv-1 can work as normal. The op amp output would be shorted to ground however (maybe throw in a 100 ohm resister in series wit the switch?) but my thinking is shorting the op-amp ouput to ground would be ok. However, throwing in a resistor (100+ ohm?) resistor in series with the switch would help? Maybe also a cap to ground?

thank you for your input! Definitely don't need any oscillation here

*edit*

maybe something like this?

[ElectricDruid]

I don't think you need a buffer for the 3.3V supply either. What's the advantage? What helps more is that 140R series resistor to limit current that flows (briefly) if the 3.3V is shorted to Ground as the jack is inserted.

I also think the 100R series resistor ahead of the CV input is perhaps a bit on the small side. Do we know how much current those inputs can take, or if the FV-1 has internal protection diodes?

Here's how I did something very similar for a PIC (5V not 3.3V, but otherwise the same situation):

https://electricdruid.net/wp-content/uploads/2018/03/FilterFXSchematicPg2.jpg

[Boner]

oh nice!

I'm guessing...

J9 is tip
J10 is tip switch
J11 is ring
J12 is ring switch
J13 is sleeve

Do we know how much current those inputs can take, or if the FV-1 has internal protection diodes?

looking into it but so far all I can find is

"The input resistance of the potentiometer control inputs is on the order of 10 megohms, and these terminals are quite
resistant to noise. No bypass capacitors should be required on these terminals. If not required, these terminals may be left
open, or if the program includes the use of a potentiometer function, it may be connected to MID if a centered
potentiometer position is desired, but an actual control is not intended in the design"

[ElectricDruid]

I'm guessing...

J9 is tip
J10 is tip switch
J11 is ring
J12 is ring switch
J13 is sleeve

Yes, exactly. I labelled most of them: T, R, S - ok, "CV" and "+5" aren't so obvious which pins they go to, but the layout on the schematic is just like the typical jack socket.

Do we know how much current those inputs can take, or if the FV-1 has internal protection diodes?

looking into it but so far all I can find is

"The input resistance of the potentiometer control inputs is on the order of 10 megohms, and these terminals are quite
resistant to noise. No bypass capacitors should be required on these terminals. If not required, these terminals may be left
open, or if the program includes the use of a potentiometer function, it may be connected to MID if a centered
potentiometer position is desired, but an actual control is not intended in the design"

Umm, doesn't help much. Nice try though. I guess that means we should be cautious.

[Boner]

https://electricdruid.net/wp-content/uploads/2018/03/FilterFXSchematicPg2.jpg

So how do you deal with the voltage drop from the 2.2k resister with the CV voltage? Do you just scale up the read voltage in your code to cancel it out and bring the signal back up to "5 volts"?

[ElectricDruid]

https://electricdruid.net/wp-content/uploads/2018/03/FilterFXSchematicPg2.jpg

So how do you deal with the voltage drop from the 2.2k resister with the CV voltage? Do you just scale up the read voltage in your code to cancel it out and bring the signal back up to "5 volts"?

There's no significant voltage drop from a series resistor if it's feeding a high impedance. Granted, with the cap there there's perhaps a bit of loss, but it's not anything you notice - and the *point* of that is "loss at high frequencies" - it's a filter!. Anyway, there's a "Depth" control on the FilterFX itself, so you can just adjust to taste.
There's nothing special done in the code that's any different from what happens when the pot is connected directly.