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I'm working on a design that requires a clock generator with variable pulse width. The pulse width is controlled by a potentiometer, and the clock speed is controlled by another potentiometer. It's supposed to work by using a 555 timer to generate a up-ramp wave, and then by comparing it to an offset created by the pulse width pot turn it into a variable width pulse wave.

Can anyone see any issues with this? The 555 up-ramp design is based on this article.

Here's the schematic. clock generator schematic

Lucas Henry
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  • So you have a cheap current source feeding a cap to generate the ramp that is fed to TR, THR, and DIS. (I'd like you to talk a little bit about using the Q output via the diode to the CV pin, which is usually just a cap to ground. Explain the details you see there.) You feed the output of the capacitor over to a follower opamp that feeds what amounts to a comparator without hysteresis to convert the ramp into a variable duty cycle pulse. Is that about it? You should really write more about your own thoughts here, I imagine. Both benefits and problems that you see. – jonk Sep 27 '22 at 03:49
  • Zeners + BJT follower aren't all that great at temp stability. You could add a diode to improve that detail a little given the BJT. (Cheap.) But I think you might write more. That would help a lot. – jonk Sep 27 '22 at 04:02
  • @jonk I'll be honest I really don't know how the sawtooth wave generator circuit works. It's shown in the article and I ran some simulations of it to make sure that it is actually functional, but beyond that I don't know whats going on really. – Lucas Henry Sep 27 '22 at 16:40
  • Oh. So you are a hobbyist builder, not so much a designer-capable hobbyist, and you want to know if this will serve the stated purpose. The basic concepts I see look like the kinds of things I might see for a variable frequency, variable duty cycle generator. I'd do things differently, but in part perhaps because I'd like to be able to calibrate something like this and I'd like stability against temperature, parts, supplied voltage, and time. (And I might just buy the generator, instead, because they aren't horribly expensive -- but that also is a matter of one's personal situation, too.) – jonk Sep 27 '22 at 20:52
  • The current source they've set up is pretty basic. The 555's voltage thresholds are a fixed proportion of the power supply provided. This means they vary with supply voltage. Which means the current source needs to vary in similar fashion, to compensate and hold the frequency. But that current source doesn't do that because of the zener. So if your 12 V supply isn't solid (battery?) you might find the frequency changing over time as the battery becomes weaker. It will also be different, one BJT to another, or over ambient temp. There's a lot more detail we'd need (like why do you need this?) – jonk Sep 27 '22 at 20:56
  • @jonk this is for making electronic music, where the gate output is a rhythm and the sequence of voltage dividers is interpreted as a melody. By changing the clock speed I'm changing the BPM of the music, and I've calibrated the clock frequency to sit in a comfortable range for this. – Lucas Henry Oct 06 '22 at 17:41
  • @jonk By changing the duty cycle of the gate output I'm also changing the duty cycle of the gate outputs, which will affect signals generated by these gate outputs. If we imagine something like an low-pass filter with variable slope for rising and falling that is then connected to a voltage-controlled amplifier, it would behave very differently if sent a pulse versus a 50% duty cycle gate, and this is something I'd like to control from within the sequencer. – Lucas Henry Oct 06 '22 at 17:41
  • Sounds to me as though precision, accuracy, and repeatability (and possibly support for the entire duty cycle range of 0% to 100%) are all very important. Would that be correct? – jonk Oct 06 '22 at 20:16
  • @jonk I think the main thing for me is just the duty cycle aspect. This design will connect with an array of other modules and those already have issues with precision and accuracy. Personally I think as long as it can output a clock and I can change the duty cycle I'm happy. – Lucas Henry Oct 08 '22 at 23:56
  • I was thinking about temperature drift, part variations, and time drift. I've spent my entire life (okay, that's exaggerating only a little bit) as part of teams making measurement instrumentation and closed loop control systems. It's way more difficult to develop a design that "just works well" every time you use it over the years. Calibration is still needed every so often. But no one wants to twiddle dials every time they use something because it's a hot day today and yesterday it was lots cooler, for example. See [here](https://electronics.stackexchange.com/a/310500/38098). – jonk Oct 09 '22 at 00:03

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