can somebody answer me? i am asking if it is normal that the frequency changes(with frequencymeter) while im turning my potentiometer

Shouldn't vary that much,

adjusting duty cycle should not affect the frequency

As has already been said - No, it shouldn't. Not much anyway. And to understand why you're seeing what you define as an unusual amount of variance - - -

The problem is, you haven't posted a schematic of your circuit.

You've gotten an answer. If you want a better answer - post a schematic please.

 

This the circuit you are using ?



What is your goal for freq accuracy/stability versus the duty cycle control ?

1%, 10%, ????

Whats your input signal look like, is it a logic signal or analog or ????

You need to indicate to the forum your performance goals so we can give you
good advice.



Regards, Dana.

 

hello ive just done it! ive give a schematic on post15!
waiting for explanation!
with this circuit i have a 75% duty cycle but (again) my frequency vary and i want to know (why)! secondly i want to obtain (if possible) a 99% dutcycle!
best regards!

 

Why not use a single chip, tried and true circuit?
Max.

 

Looking at the schematic in post 15...

The timing cycle on 555 two cannot end while 555 one output is still low, so you may be seeing a timing mismatch if 555 two's cycle is longer than 555 one.

In other words the one shot cycle MUST end before the astable goes low.

Just a wild guess....

 

hello ive just done it! ive give a schematic on post15!

First - forgive me for missing something. But what you posted in post 15 was the same exact circuit posted in post 14, posted by @ericgibbs; only, in post 15 you (or someone) made a correction to a part of the drawing. Before post 14 I don't see a schematic posted by you. I assumed you were discussing Eric's schematic and not your own. So forgive me for missing something. In post 13 @crutschow said you haven't posted a schematic (yet) so I assumed Eric's schematic was something you were quoting, but you didn't say it was "Your" schematic.

Crutschow said the circuit posted in post 12 (actual quote, he said #2) would work. In post 19 MaxHeadRoom stated that post 6 would be best; which is basically the same circuit I posted. In post #22 there's still confusion as to what you are building because @danadak asks if "This" is the circuit you're using. Finally, @ElectricSpidey is confused with the dual 555's and concludes his comment with "Just a wild guess…."

So far we don't know for sure what circuit you're building. We can't sit here and guess as to what you have when you don't post your own circuit. If you are using the exact circuit as Eric's schematic, then please state so clearly. That way we can stop making assumptions and wild guesses. We DO like helping, but it requires clear answers and documentation on your part as well.

Why you're using dual 555's - you're the engineer. Ours is not to question why but to help you understand how and why your circuit is not functioning the way you anticipated it would. For that we need to see your circuit. Not someone else's. Finally, we may offer advice such as in post #6 and #12, but nobody is going to twist your arm. In the final analysis, you ask "if it is normal that the frequency changes(with frequencymeter) while im turning my potentiometer" That question has been answered.

 

I'm not confused, I'm just not sure that what I posted is the problem.

 

@ElectricSpidey sorry, don't mean to say you're confused. What I should have said was "Even Electricspidey is unsure of what circuit you're using."

 

The TS posted a circuit earlier, and then deleted it. He/she has been editing his/her post, confusing the whole lot of us

 

iimagine
why? post 15 is the schematic! (the first one had a capacitor connected to pin 6 of the 741!

 

ElectricSpidey
???

 

TONYR 1084
ok Tony so the first 555 is a reference frequency to the second 555 who is the pwm output controlled by 741 voltage follower! i
have tested it on e scope and i have 75% dutycycle and on a frequency meter it vary from some hz to the (reference frequency) my question was why! (no answer) and the question was clear!
the same solution like (crutsshow) said can work with only one 555 with a 741 for varing the pwm, i allready no that in monostable mode!
this was not my question!
the schematic on post 15 is the schematic and not an other one!
have a nice day!

 

The circuit operation….

The astable sets the frequency.
The one-shot provides the output pulse.
The 741 changes the one-shot’s threshold via the CV, changing the output pulse length.

Therefore the one-shot must operate within the period of the astable or it will miss a trigger and the frequency will change.

At least that’s my theory.

 

Or... Just get one off ebay for $3.00
Max.

 

Hello,

Have a look at the sections 4 and 5 of the fairchild 555 datasheet.

Bertus

 

ElectricSpidey
good for you!

 

bertus
and?

 

my question was why! (no answer) and the question was clear!

Your question (post #1) was

Hello everybody i am configuring a 100% duty cycle with two 555 and i have a frequency who vary from 30 hz to 635HZ and i am asking if it is normal that the frequency changes(with frequencymeter) while im turning my potentiometer (i have a scope and i have 100% dutu cycle)
thank you

No. It's not normal.

 

@MATT838383

The circuit operation….

The astable sets the frequency.
The one-shot provides the output pulse.
The 741 changes the one-shot’s threshold via the CV, changing the output pulse length.

Therefore the one-shot must operate within the period of the astable or it will miss a trigger and the frequency will change.

At least that’s my theory.

After reviewing most of the responses - I believe this answers your question. I think any further involvement by me will only be agitating. Sorry to have upset you.

'av'a g'day mate. Stay healthy.

 

i want to obtain (if possible) a 99% dutcycle!

What minimum duty-cycle do you want?

Below is the LTspice simulation of your circuit with modified values, for a modulation input of 0 to 12V.
The output frequency does not change with PWM duty-cycle as expected.
The duty-cycle varies from about 5% to 92%.

Note the right 555 RC timing values used.
The circuit will not work with the values you have as the one-shot period was too long (about 33 times).
Also you don't want a capacitor at the 741 output as that may cause it to oscillate.

If you want a full 0% duty-cycle (fully off) to 100% (fully on) there are circuits using a single op amp or comparator package (e.g. the LM324 or LM339) that will do that.