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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
pulse width modulation question
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teddexter



Joined: Feb 27, 2009
Posts: 9
Location: Leicester

PostPosted: Thu Oct 01, 2009 6:08 am    Post subject: pulse width modulation question Reply with quote  Mark this post and the followings unread

Hi,
Am looking to turn an oscillator from a 40106 or 4093 into one where I can control the pulsewidth either with a pot or vactrol or some other method of voltage control.

It takes ages to search for specific stuff on these forums, but I expect its already covered somewhere. Looking for a simple schematic that covers just that aspect rather than a real bells and whistles oscillator, so I can adapt my current ideas to suit.

Just doing an oscillator with variable resistor between the in and out and a 0.1uf cap going to ground.

Cheers
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slacker



Joined: Nov 18, 2007
Posts: 301
Location: England
Audio files: 11
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PostPosted: Thu Oct 01, 2009 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

The first schematic in this thread http://www.electro-music.com/forum/topic-23878.html has a pulse width control. If you removed the polarity switch and fed a control voltage into pin 1 of the pulse width pot in theory you could modulate the pulse width.
You could also replace the pot with a vactrol or put a vactrol in series with the pot and do modulation that way.

For another way to do it have a look at the thing I posted in this thread http://www.electro-music.com/forum/topic-35512.html, ignore the opamp and the rest of the stuff. The pot connected to the 2 diodes controls the pulse width. The only problem with this is that the pitch changes as well as the pulse width.
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teddexter



Joined: Feb 27, 2009
Posts: 9
Location: Leicester

PostPosted: Fri Oct 02, 2009 3:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Slacker,

Thanks once again. I'm drawn to the second option for some reason. So its a couple of diodes and a pot. Is there a preferred diode type. Thats the sort of thing I was hoping to find. I have some 1N914s and some germanium diodes knocking about. Would it work ok off 12v as thats the power supplies I have. Well I'll just try it rather than the twenty questions!Smile Report back soon.

Have we also tried the sample and hold from a CD4016 in the Don Lancaster. Thats what I want a pulse for as at the moment I feed it a square wave to open/close the switch. Effective, but its only every other note that is "sample and held" because the switch is held on for half the time, so I figure I need a shorter pulsed input to fix that. Does that make sense?

Cheers
Ted
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teddexter



Joined: Feb 27, 2009
Posts: 9
Location: Leicester

PostPosted: Fri Oct 02, 2009 3:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah just realised that you say the freq changes with the pulswidth on the second (must read properly) so thats a little off putting as not the intended result for me. I'll try both anyway.
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Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 6221
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PostPosted: Fri Oct 02, 2009 4:08 am    Post subject: Reply with quote  Mark this post and the followings unread

teddexter, try this thread:

http://www.electro-music.com/forum/topic-35512.html

It uses a pot with two diodes in the feedback loop to control duty cycle.

Les

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slacker



Joined: Nov 18, 2007
Posts: 301
Location: England
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PostPosted: Fri Oct 02, 2009 9:22 am    Post subject: Reply with quote  Mark this post and the followings unread

teddexter wrote:

Have we also tried the sample and hold from a CD4016 in the Don Lancaster.


As it happens the rest of that schematic snippet I posted is exactly that Smile
To get a thin pulse to drive the CD4016, actually CD4066s in my case, I'm basically using the first method I posted, but without the pulse width pot, just the resistor (R6) connected to the positive supply. That gives you a thin positive going pulse.

The idea, once I get all the bugs worked out, is that you'll have 3 triangle, square and possibly ramp LFOs, each one using 2 stages of a CD40106 that also drive 3 sample and holds.
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teddexter



Joined: Feb 27, 2009
Posts: 9
Location: Leicester

PostPosted: Tue Oct 06, 2009 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Inventor. Slacker already posted that link though. Grateful for the help anyway though!

I liked the bugs circuit because you can adjust the pitch separately so you would get a pulse I could use for my s&h as well as for audio. However the pulse looks a bit gnarly perhaps due to capacitor's influence

[img]http://www.crowphonics.com/images/pulsewave.bmp[/img]

This means it doesn't send the s&h switch into closed state for long enough so is not behaving right. Will try the method of reversing the polarity but I suspect same would occur. I could perhaps clean up with a comparator circuit, but would need to use op amp I guess (not so religious about lunetta rules if I do this). Or could try the diode method just for the s&h and the cap method for a VCO (I like the gnarly sound so thats not a problem there).

Updates to follow later.

Cheers and Thanks
Ted
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