PWM external CV input mod

[Strategy]

Hopefully this isn't redundant, there are tons of posts on PWM but I couldn't find my exact answers in the old threads.

I built PWM a while back and used a layout from this forum that doesn't have an LFO. I added an expression pedal input for the 500K pot so that the PW could be modulated by foot. Works well with the right expression pedal!

I would also like to add an external CV input so I can modulate the PW from my modular synthesizer equipment (and other CV generating stuff, moogerfoogers for example) - most of which puts out +5V control voltages.

I assume I would just wire the CV jack directly to pin 3 of the 40106 in the PWM, can anyone confirm this?

And, would it be recommended to add a zener diode in series as a protection against over-voltaging the logic chip? I know these can be touchy so I'm wondering if it's unwise to just be piping voltages into it!

Thanks for any help!
STRATEGY

[earthtonesaudio]

The PWM works by using a Schmitt trigger after an RC filter.  The "R" is key to this function.  If you want to use a CV to change the cutoff frequency you need to turn a voltage into a resistance somehow.

Personally I would use one of the spare gates to make a high-frequency PWM to synthesize a resistance with an external transistor.  

like this
In this sim the "CV" is the 25Hz waveform.  I've represented one of the Schmitt trigger gates with a 555.  This drives a transistor which is used as a PWM-variable "resistance" to ground.



There are other ways to turn a CV into a variable resistance, such as LED/LDR combos, but it's called PWM so why not use PWM? 

[Strategy]

The PWM option is a little bit over my head...I understand using an LED/LDR or vactrol type solution, although I haven't found a good tutorial/graphic demonstrating the wiring.

If anyone has a reference to a 'proper' how-to on this, that would be appreciated!

Strategy

[earthtonesaudio]

There's also the transistor method.  Even simpler than the LED/LDR method.

Here's a link showing both methods and some suggested starting values. 
LINK

[Strategy]

thanks for that simulation!!

question: what sort of transistor is best used for this application?

thanks again for your help!

Before I go subjecting my precious pedals to such experiments in voltage controlled resistance, I'm going to apply some opto or transistor CV input mods to my Music From Outer Space "Weird Sound Generator." - I built it cheaply and feel it should be a good vehicle for experiments. (also uses logic chips.)

Strategy

[Strategy]

I used an early DIY project of mine - the Music From Outer Space Weird Sound Generator (self-etched PCB version) - as a guinea pig for the external voltage control input. I used a vactrol per earthtonesaudio's simulation example!

So great to finally implement this. I'd seen it as part of bigger circuits but never had a clear instruction on how to do it as a mod.

I figure many of my pedals will now be given CV input mods, including PWM.

I only had 4 VTL5C3 vactrols so I only could do 4 of the 5 controls I desired to make voltage controllable. It makes this relatively stiff CMOS based noise/beep machine into a thing capable of complex, animated drones that can interact with my modular synths, moogerfoogers, etc.

The pic is a phone pic, so not great. Will try to post audio this week.

Strategy

[earthtonesaudio]

I didn't see you transistor question until just now, any general purpose NPN should work... for future reference anyway.  Glad to know you got it working with the LED/LDR!  Sounds fun.

[Strategy]

I will probably try your transistor suggestion. I can see that on some commercial pedals that are tightly packed into the enclosure, adding a CV input using LED/LDR or vactrol will not be physically possible due to space.

thanks again for your help, nudging me towards something I'd been wanting to try for a long time!

I didn't see you transistor question until just now, any general purpose NPN should work... for future reference anyway.  Glad to know you got it working with the LED/LDR!  Sounds fun.