555 astable osc. Duty cycle questions

[Dr. K]

I built a 555 based oscillator according to the schematic attached. Recently I picked up an o-scope, and I hooked up the probes to take a look. I was surprised to see that the duty cycle for the oscillator is very high. It must be around 90%, and doesn't seem to change much depending on the frequency of the oscillator.

I was under the impression that the duty cycle hovered somewhere around 50%, moving a little bit as a function of frequency.

I was doing some reading on the subject, and looking in to using anti-parallel diodes and resistors to tweak the duty cycle.

The circuit seems to function fine. The frequency can go from metronome to extremely high. I think it's working properly. It was just the duty cycle that seems like it shouldn't be so high.

---

555 oscillator.png
Description:
Filesize:	14.72 KB
Viewed:	108 Time(s)

---

---

20250503_102023.jpg
Description:
Filesize:	2.16 MB
Viewed:	14 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

[Cynosure]

I think that Phobos mentioned this in your previous post when you asked about 555 vs 4046.

https://electro-music.com/forum/post-456945.html#456945

You could try reading through this thread about PWM:

https://electro-music.com/forum/topic-56412.html

[blue hell]

Ow, and when the small pulse width - heh -or large - whatever - is a problem - make the frequency twice as high (as in halve the capacitor) and divide the output by two with a flip flop thing or a binary divider .. it will be a perfect 50/50 square then.

[PHOBoS]

You should be able to change the duty cycle by changing the ratio between the 2 resistors (in the schematic the 1K and 500K pot)
This is because it charges the capacitor through both resistors but discharges through only one of them (I'll let you figure out which one).
You could play around on a breadboard and a scope (good practice), use a simulation or mess around with one of those 555
calculators like this one: https://ohmslawcalculator.com/555-astable-calculator


There are ways to get a 50% duty cycle out of it by making use of the output as part of the oscillator, for example this circuit:

which you can find here: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html


or you could divide it as Jan mentioned but then you might as well just use a CD4060 or CD4046.

[Dr. K]

I was aware that in the 555 astable, duty cycle and frequency were interdependent. But I ran through a couple of calculators I found online (DigiKey and some other places have them), and for the audible range, the duty cycles were all around 40-60%.
When I measured mine, it was (like in the image) at least 90%. I'm trying to read up on solutions to it--it looks like it can be dialed in to a large extend with diodes and a potentiometer. Not wanting to put any more controls on the user interface of this module, I'd probably go with a trim pot, and just do a "set and forget" on it.