How to include a CV/expression pedal input

[pimpifax]

Hello.

I´m not only new in this forum. Also in (building) electronics. As well english isn´t my mother tonge, so i hope my writing isn´t to bad...

So i already build this PWM pedal which works quite fine.
My second project will be this analog bit crusher. But my Problem is that i want to manipulate the sound while playing. Which means i want to include an input for an expression pedal or better an CV (controlled Voltage) input that is also used with synths. I already have a midi 2 CV converter and this would allow me also to use a recorded process. So:

- Is it possible to include an expression pedal better a CV input?
- Is CV and exp pedal the same anyway?
- Can someone post me a link, a Schematic, or an instruction how i have to do it or how i should be done?

Thanks.

[Strategy]

Expression and CV are not the same. I use synths and pedals with voltage control, ones I built as well as moogerfoogers and things like that that work well with these.

Expression pedal is a passive form of remote control. It's essentially a potentiometer inside a treadle (wah style) pedal. To add an expression control to a pedal control, you put a stereo switching jack between the pot you want to control and the board. When you plug in your expression pedal's cable into this jack, it disconnects the pot, putting the expression pedal into the circuit instead. There are a LOT of instructions on ways to do this online; google it or try diystompboxes.com forum.

Tips: make sure that the expression pedal resistance value is similar to the pot value you want to control. Expression pedals tend to contain 10k-50k pots, some of them like Moog brand and Roland have variability options. I tried adding an expression pedal input to a 10K pot the other day and I just could not get any of my expression pedals to be close enough in resistance value. It sounded messed up. There are workarounds but it's not instant gratification.

Control voltages are actual live voltages you are introducing into your circuit from another piece of equipment. You cannot always control any pot with an external CV. Sometimes you can tap into an existing voltage control path in the circuit - say a phaser, which already has an LFO- and put your incoming voltage in parallel. You have to be careful how much "juice" (voltage) you are inserting into the circuit though, and without pots to scale your voltage etc. it won't necessarily have the desired results. This requires some expertise.

One trick to adding CV's to pedals is to use optocouplers, which in some of the forums you'll see just referred to as "vactrols". Vactrols are a specific brand of optocoupler. They are expensive but useful. There are equivalent brands as well.

Inside is an LED that faces towards a photo resistor and its all locked into a completely sealed little box. You can create a control voltage input - sometimes - by putting your CV into the LED side of the opto, and then from your resistor side of the opto, send that to the pot/control you want to control via CV. For example, if you have a delay time knob you want to control from an analog sequencer, you could try applying the photoresistor side of the opto to the center lug of the delay time pot and see what happens.

What is happening is that you are creating a "voltage controlled resistance" - basically, a kind of potentiometer -- your external voltage lights up the LED, and the photo resistor responds to the brightness of the LED with a corresponding resistance value. This is the principle behind many popular pedals that have this built into the circuit, like envelope filters and some LFOs in phasers etc. including many classic designs like Mutron III (Nautilus).

TIPS:

- creating voltage controlled resistance with optos and plugging them into potentiometers will not always work. The voltage controlled resistance created by the incoming CV and the opto should again be scaled appropriately to the pot. I've had a hard time getting CV input to control big resistance values like 250K and upwards. On the other end of hte spectrum, sometimes the variable resistance is too much and you need to use another pot to attenuate it. I had to do this adding a voltage control to the delay time pot of my early 90s Deluxe Memory Man.

- protect your opto (they can be expensive) with a 200R resistor, in series between your jack and the LED side of the opto - you can easily fry the internal LED with a hot control voltage from a synth etc.

- you can create your own "voltage controlled resistance" setup by using your own LED and LDR (photo resistor) and then wrapping them in a little tube of heat shrink tubing! Make sure it's dark in there, no external light leak. I haven't found the right recipe for it but people enjoy experimenting with this and its cheap. This is an old DIY standby. Maybe experiment with a cheap commercial pedal you got at a garage sale or something before applying voltages to your precious projects or fancy pedals.

I've added CV's to filter pedals, delays, ring modulator, all kinds of stuff. it's really fun for interfacing the equipment to synths or for example, I will use the envelope output from a moog filter to control the delay time of a delay pedal. pretty trippy

STRATEGY

[alanp]

- protect your opto (they can be expensive) with a 200R resistor, in series between your jack and the LED side of the opto - you can easily fry the internal LED with a hot control voltage from a synth etc.

To put this in perspective, most DIY analog synth circuits run on +/- 15 volts, meaning a 30 volt range. The signal might be less than this, but that's the upper limit (probably 15v, if ground is working like I think it is.) Most guitar pedals run on +9V, meaning a 9 volt range. Guitar signal is piddling compared to synth signal.

I Am Not A Synth Dude.

[Strategy]

Alanp is correct the power supply of the circuit determines the ceiling of the voltage control. I'm making a diagram of the "voltage controlled resistance" modification

this is not just for synth dudes, highly recommended if you have pedals with modulation you can start getting them to 'talk to each other' through CV's. Gets VERY fun.

Strategy

[Strategy]

here's a diagram. I'm not an expert, there could be stuff wrong, anyone with insight please (kindly) comment and I'll revise.

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I could have used this back when I was a beginner! It will take you a few projects and some time before it makes sense.

STRATEGY

[pimpifax]

Thanks very much for your answer!!
To prevent crossposting i may want to refer to the same problem i posted here in the diystompboxes forum.