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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
cmos pwm ?
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synthesist



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PostPosted: Mon Jan 07, 2013 5:11 am    Post subject: cmos pwm ?
Subject description: maybe with transistor
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hey !

To change the pulsewidth on a 40106 oscillator you only need to place a diode on the input of the inverter, the other end goes to a variable resistor with its wiper placed to the inverter output.

but how can one modulate the PW? Did anyone try it with a resistor yet?
or is there any other way?
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RingMad



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PostPosted: Mon Jan 07, 2013 8:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Using the following schematic, one can control both the mark & space / duty cycle parts of the square wave. LEDs or diodes like 1N4148 work. You can also use a 4093 NAND gate instead of the 40106 (shorting the 2 inputs together).

To modulate, I guess you could replace the pots with opto-isolators ("Vactrols") LDR/LED combination.

James.


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PHOBoS



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PostPosted: Mon Jan 07, 2013 8:48 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm, using a shiftregister or divider comes to mind,. but not sure how yet. thinking
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PostPosted: Mon Jan 07, 2013 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

A & B could be a single dual pot with the two sides wired opposite each other as well. That would give a pretty smoothly changing transition of pulse width.
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PostPosted: Mon Jan 07, 2013 9:38 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
A & B could be a single dual pot with the two sides wired opposite each other as well. That would give a pretty smoothly changing transition of pulse width.

that works, but the frequency will be fixed. You could add another pot in series but because of the pot for the
PWM control the range will be limited. And in turn it will influence the PWM control.

That's a nice thing about the standard saw/triangle + comparator combo,.
you have indepent frequency and pulse width control.

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PostPosted: Wed Aug 28, 2013 12:37 pm    Post subject: gated LFO with pulse width control Reply with quote  Mark this post and the followings unread

ok here is something I just came up with. I wanted to make 1 LFO using
a 4093 but didn't want to waste the other gates so I wondered if I could
add some pulse width control to it.

The first part around U1b is a standard (gated) oscillator but because of
the diode parallel to the potentiometer it creates a pulse. (the output is
high most of the time and the pulse is a low voltage). U1a inverts and
buffers the output. The next part, which looks similar to the oscillator,
adjusts the width of the pulse. Because of the diode the capacitor is
charged instantly and the discharge time is set with the potentiometer.

This creates a problem though, if you set the discharge time longer then
the time between pulses the output wouldn't oscillate anymore which is
what the connection between U1b and U1d is for. This will make sure that
the output still changes.

The circuit isn't perfect. I noticed that if you set the discharge time just
right (or wrong, depending how you look at it) it will toggle between a
short and a long pulse. Also the gate works a bit unusal, and it can be
used as a pulse lenghtener, and I think there is also some delay. Cool

note: replacing the 10uF with a 4.7uF gives a nicer control for the faster
settings, but you won't get the maximum on time at the lowest speed.


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Cynosure
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PostPosted: Wed Aug 28, 2013 4:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is how I do it - especially if my oscillator is a 40106. You can replace the passive filter with VCF for CV control.

I have only tested it with a square wave, but it should work reasonably well for any waveform that spends half the cycle above the midpoint.


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PostPosted: Thu Aug 29, 2013 7:06 am    Post subject: Reply with quote  Mark this post and the followings unread

http://www.electro-music.com/forum/topic-55957.html
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Cynosure
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PostPosted: Thu Aug 29, 2013 10:22 am    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
http://www.electro-music.com/forum/topic-55957.html

I guess great minds think alike Smile

It is a pretty simple idea. I am sure that we aren't the only ones who thought of it.
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PostPosted: Thu Aug 29, 2013 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Cynosure wrote:
It is a pretty simple idea. I am sure that we aren't the only ones who thought of it.

check Tim Escobedo's PWM Wink

problem with this method is that it doesn't work with an LFO, or at least you won't get a very width pulse out of it.

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jonasx26



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PostPosted: Fri Aug 30, 2013 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi! This might work as an alternate approach. Pulse width totally independent of frequency but the output frequency will be divided by 10.
Heavily inspired by something like a 80s "Allt om Elektronik" (swedish Elektor) article, maybe.. Can't remember where it's from. Anyways, I didn't come up with it. Drawn from memory, presented AS-IS but I'm pretty sure it will work.
I think the original used a NAND/NOR cross-coupled latch instead of the 4013 D-type. But this should be functionally equivalent.
OUT1 is the variable pulse width output.
OUT2 is a constant 50/50 square wave. Essentially the input divided by 10.


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trav



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PostPosted: Sun Sep 01, 2013 2:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey, I like that solution! sounds like what PHOBoS was thinking in his first post. use a multiplexor instead of S1 and you're in business.
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PostPosted: Mon Sep 02, 2013 7:30 am    Post subject: Reply with quote  Mark this post and the followings unread

trav wrote:
hey, I like that solution! sounds like what PHOBoS was thinking in his first post. use a multiplexor instead of S1 and you're in business.

It is indeed similar to my initial idea. I think I actually have a version of that circuit in a book somewhere that uses 2 4017 counters,
so you can set the dutycyle with 2 rotary switches in steps of 1% instead of 10%. Of course for audiofrequencies it's nice if you can
sweep it without steps, but using a mux might create nice effects.
Idea control the mux(es) with a random generator for random pulsewidth Very Happy

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PostPosted: Wed Sep 11, 2013 7:49 am    Post subject: PWM with a single XOR gate
Subject description: can easily be added to a CD4046 VCO
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here's a method which I thought up while trying to sleep. (I count electrons not sheep).

I have a CD4046 VCO wired up with it's output (pin4) connected to the Internal Phase Comparator I (pin3), which is just an XOR. And at some
point I wondered what happens if you connect the output of the VCO to the signal input (pin14) which is the other input of the XOR. Well that
would mean that the inputs of the XOR are just shorted together and therefor it's output will always stay low. So not very intersting,..

But that got me thinking,. what if I delay one input ? Then the inputs would be different from each other for a short time and therefor it's output
will also be high for a moment. In other words I can control the pulse width. Very Happy A simple adjustable delay can be made with a potentiometer and
a capacitor, and by adding a diode parallel to the potentiometer, the capacitor will get discharged instantly when the input goes low and so will
the output.

I did expect one problem, that is without a schmitt trigger input such a simple delay might not work so well, and cause unwanted oscillations.
So I just breadboarded it but I had to use an external XOR (CD4070) because I have the VCO build on a PCB and only made the XOR output
available. Well, it works perfect. Laughing And the diode seems to take care of the unwanted behaviour I expected, without it I do see and hear some
extra oscillations.

The maximum 'on-time' is the same as the input signal and since the CD4046 has a pulse width of 50% I can't set it longer than
that. So with the right capacitor (dependent on the frequency) it's adjustable from 0 to 50%. But soundwise making it longer would sound
the same anyway. a 500K pot and 10nF capacitors seem to work nice.


fun fact: when I tried to adjust the circuit to work with NAND gates, I ended up with the same circuit I posted before. Cool


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PostPosted: Wed Sep 11, 2013 12:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

A very ingenious and elegant solution. Nice work Phobos! thumright
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PostPosted: Thu Sep 12, 2013 9:31 am    Post subject: Reply with quote  Mark this post and the followings unread

pulse width % will vary with frequency. these circuits provides constant pulse duration, so the timbre of the notes will change. with a cap in line with a pot to ground bass notes will sound very thin and reedy, while the higher you go the closer the pulse width percentage will be to 50%. close enough for jazz, though, amirite?
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PostPosted: Thu Sep 12, 2013 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Psyingo wrote:
pulse width % will vary with frequency. these circuits provides constant pulse duration, so the timbre of the notes will change. with a cap in line with a pot to ground bass notes will sound very thin and reedy, while the higher you go the closer the pulse width percentage will be to 50%. close enough for jazz, though, amirite?

Good point. It is probably ok for lunetta randomness or if you stay within a certain range, but will sound different as the frequency changes. I guess that is why a comparator on a saw wave is the most common method that I have seen.

It's just a pain to get a saw from a lunetta. The best method I have tested is a R2R ladder on all the outputs of a 4040, but that either requires a lot of resistors or a couple of (relatively) expensive ladder IC's.
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PostPosted: Thu Sep 12, 2013 10:47 am    Post subject: Reply with quote  Mark this post and the followings unread

could always try this:
you know how the quote goes... "Don't fear the opamp"

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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PostPosted: Thu Sep 12, 2013 10:58 am    Post subject: Reply with quote  Mark this post and the followings unread

yes I think you're correct. The pulse duration is set at a fixed time so if the frequency changes the pulse width will change too. And the pulse
duration will be a smaller % of one period at lower frequencies then at high frequencies. Of course without resquaring the signal it would just be a
passive highpass filter which would have a similar effect on the sound but it's not the same I think. (something with harmonics). So the thin and reedy
sound is in this case a result of changing the pulsewidth.

The circuits I came up work slightly different and work well with low frequencies. At least the first one was designed as an LFO the other one is
just an easy way to add pulse width control to a 4046 but can be used standalone and also works well with an LFO. But the pulse width does also
very with frequency. I don't think there is anyway around it unless you go for a different approach like a sawtooth oscillator + comparator.

edit: I think I need to type faster, this was a reply to psyingo's first post

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PostPosted: Thu Sep 12, 2013 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I would add a diode to create a saw instead of a triangle, else you don't just get pulse width control but phaseshifting aswell (unless that's a wanted feature).
well actually I'd just grab a quad opamp if I go that way, and leave out the 40106 completely.

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Psyingo



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PostPosted: Thu Sep 12, 2013 11:22 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

I would add a diode to create a saw instead of a triangle, else you don't just get pulse width control but phaseshifting aswell (unless that's a wanted feature).
well actually I'd just grab a quad opamp if I go that way, and leave out the 40106 completely.


very very true. many ways to do it, and i hadn't considered the phasing issue. The OP referred to a 40106 oscillator, but you could do it any way. many ways to do the same thing.

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