555 astable multivibrator-vary duty cycle/maintain freq

Thread Starter

spuddo

Joined May 2, 2013
14
Hi
This is a cct from a forum known as "quoro" , the claim was that with two tweaks of this cct you could vary the duty cycle but maintain frequency!!

https://www.allaboutcircuits.com/uploads/thumbnails/astable-555-circuit.png?v=1452314372856

  1. Make the R1 and R2 the resistance of a single pot with fixed value (R1+R2), but with a sliding tap. Pin 7 will be connected to that slider tap.
  2. Next, Fix a diode like 1N4001 between the pins 7 and 6( or 2) pointing downwards.
I built this cct and the end result was , i could vary the duty cycle from 1% to 99% , but the frequency varied by a factor of 1.5.
The pot was 2kohms and C was 10 nanofarads , Vcc was 9 volts.

Any chance this can work with a few more tweaks?

Best regards
 

danadak

Joined Mar 10, 2018
3,630
Could be my eyes but the waveform examples seem to me to be showing
freq change with duty cycle.

What is your goal for freq stability ?

A simple ATTINY85 can absolutely control duty cycle over a broad range w/o
affecting frequency. One chip, just pot, no other external components unless
you need xtal accuracy for the frequency.

You can program it in C or Bascom Basic.

https://www.mcselec.com/index.php?Itemid=41&id=14&option=com_content&task=view


Regards, Dana.
 

AlbertHall

Joined Jun 4, 2014
8,551
RV1 is variable, how can this maintain a "fixed" freq? I really need an OScope.
RV1 will very slightly change the frequency but its principal effect is on the mark space ratio. Using a higher value for RV1 will give a lower frequency but the same mark space ratio change.
 

crutschow

Joined Mar 14, 2008
23,542
RV1 is variable, how can this maintain a "fixed" freq?
The value of RV1 (total pot resistance) is not variable, only the wiper changes position, which mainly affects the duty-cycle but with only a slight effect on frequency.
It's the total pot resistance that affects the frequency.

Think of it this way:
As the pot wiper changes position, the capacitor discharge time will increase (decrease), while the charge time decreases (increases) by a proportional amount.
In either case the sum of the total charge-discharge time stays essentially constant, giving a constant frequency.
 
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Thread Starter

spuddo

Joined May 2, 2013
14
Could be my eyes but the waveform examples seem to me to be showing
freq change with duty cycle.

What is your goal for freq stability ?

A simple ATTINY85 can absolutely control duty cycle over a broad range w/o
affecting frequency. One chip, just pot, no other external components unless
you need xtal accuracy for the frequency.

You can program it in C or Bascom Basic.

https://www.mcselec.com/index.php?Itemid=41&id=14&option=com_content&task=view


Regards, Dana.
 

Thread Starter

spuddo

Joined May 2, 2013
14
Hi all , thanks for your replies.
I'm an old fart who is doing this to relearn what i started in the 60's , changed careers for 55 + years , so there is no particular goal in mind , just the pleasure of making stuff and hopefully learning .
Have built the cct that Albert Hall provided.
Works , but frequency changes with duty cycle change
moving duty cycle from 1% to 99% and 99% to 1% , the frequency doubled.
Any ideas.
kind regards
 

danadak

Joined Mar 10, 2018
3,630
What is your target goal for freq stability, tolerance, and accuracy ?

Have you ever done any programming of micros, Basic, C, ASM ?

Regards, Dana.
 

AlbertHall

Joined Jun 4, 2014
8,551
Hi all , thanks for your replies.
I'm an old fart who is doing this to relearn what i started in the 60's , changed careers for 55 + years , so there is no particular goal in mind , just the pleasure of making stuff and hopefully learning .
Have built the cct that Albert Hall provided.
Works , but frequency changes with duty cycle change
moving duty cycle from 1% to 99% and 99% to 1% , the frequency doubled.
Any ideas.
kind regards
Were you using the NE555?
If so you may get better results using the CMOS version, LMC555.
 

iONic

Joined Nov 16, 2007
1,650
spuddo,
Looks like you and I are after a similar circuit. My post is HERE. It would be used to control freq/duty-cycle for three separate channels of LED lighting(Red, Blue, Green.) My circuit began like the one eetech00 posted above, but is morphing into an adjustable, but hard FIXED freq. with variable duty-cycle using a quad op-amp. In your situation you may only need a dual op-amp. danadak's push for microprocessors is the real solution for today and in the future, but if you do not already have a programmer and know some programming, it could lead you down a lengthy path for project completion.....unless you are really diligent. All my opinion, mind you. It's actually hard to believe that this type of device isn't easily available on the market today for next to nothing from ebay, but I haven't seen it. Surprises me.
 

Danko

Joined Nov 22, 2017
826
Simple select resistance R2 for stable frequency.
R2 may be pot 22k for tuning stability in PWM range.
upload_2019-2-8_21-52-42.png
 

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AlbertHall

Joined Jun 4, 2014
8,551
spuddo,
Looks like you and I are after a similar circuit. My post is HERE. It would be used to control freq/duty-cycle for three separate channels of LED lighting(Red, Blue, Green.) My circuit began like the one eetech00 posted above, but is morphing into an adjustable, but hard FIXED freq. with variable duty-cycle using a quad op-amp. In your situation you may only need a dual op-amp. danadak's push for microprocessors is the real solution for today and in the future, but if you do not already have a programmer and know some programming, it could lead you down a lengthy path for project completion.....unless you are really diligent. All my opinion, mind you. It's actually hard to believe that this type of device isn't easily available on the market today for next to nothing from ebay, but I haven't seen it. Surprises me.
If you are controlling LED brightness using PWM then the frequency is pretty irrelevant so the simplest '555 circuit will do the job.
 
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