pulse generator

[samjesse]

How about this, credit goes to bertus at allaboutcircuits.

The 1M pot will change the frequency.
The 10K pot will change the duty cycle.

 

[Roff]

How about this, credit goes to burtus at allaboutcircuits.

The 1M pot will change the frequency.
The 10K pot will change the duty cycle.

I was going to say that Bertus (note spelling) has a good solution. I personally would use a real comparator instead of an op amp (and a poor one at that), but otherwise, it's a good solution.

 

[BrownOut]

That's true, but the frequency is not changing. The duty cycle is simply going to 100%. The osc frequency is still the same.

We're interested in one signal that is frequency and duty cycle adjustable. If the frequency is such that full duty cycle adjustment > 100?, which is very possible, then the final output's frequency is affected.

 

[Roff]

We're interested in one signal that is frequency and duty cycle adjustable. If the frequency is such that full duty cycle adjustment > 100?, which is very possible, then the final output's frequency is affected.

You would probably want to set up the limit resistors on the ends of the duty cycle pot (10k) so that it goes from something like 1%-99%, or 10%-90%, or whatever, so that the frequency never goes to zero, which it will if the pot range is excessive on either end.

 

[BrownOut]

If, for example, you have a duty cycle pot set up to be 10-90% at f=1khz, then at f=2khz, full adjustment gives you 20% to 180%. Anything over 100% drives the output signal period to the one-shot, and screws up the output frequency.

 

[bountyhunter]

You would probably want to set up the limit resistors on the ends of the duty cycle pot (10k) so that it goes from something like 1%-99%, or 10%-90%, or whatever, so that the frequency never goes to zero, which it will if the pot range is excessive on either end.

I never had a problem with it going to 100%. All the benchtop signal generators can go to 100% if you turn the knob.

 

[Roff]

If, for example, you have a duty cycle pot set up to be 10-90% at f=1khz, then at f=2khz, full adjustment gives you 20% to 180%. Anything over 100% drives the output signal period to the one-shot, and screws up the output frequency.

It's not a oneshot, it's a comparator, slicing the triangle wave generated by the 555. If the comparator were ideal, the duty cycle would be absolutely independent of the frequency.

 

[bountyhunter]

If, for example, you have a duty cycle pot set up to be 10-90% at f=1khz, then at f=2khz, full adjustment gives you 20% to 180%. Anything over 100% drives the output signal period to the one-shot, and screws up the output frequency.

Which is why (if you wanted to prevent 100% DC) you simply go to maximum frequency and dial in the R-C on the one shot to go to 95% duty cycle. Then, at lower frequencies, the maximum duty cycle is less than 95%.

The frequency would be wherever you set it, and the duty cycle would still be adjustable up to the maximum limit which would depend on the particular frequency.

 

[Hero999]

How about this, credit goes to bertus at allaboutcircuits.

The 1M pot will change the frequency.
The 10K pot will change the duty cycle.

What's wrong with my idea?

Did you even look at it?

Why have two ICs when you only need one?

 

[Roff]

What's wrong with my idea?

Did you even look at it?

Why have two ICs when you only need one?

Looks better than Bertus's circuit to me. Same principle, fewer ICs (but a few more resistors).

 

[Roff]

Which is why (if you wanted to prevent 100% DC) you simply go to maximum frequency and dial in the R-C on the one shot to go to 95% duty cycle. Then, at lower frequencies, the maximum duty cycle is less than 95%.

The frequency would be wherever you set it, and the duty cycle would still be adjustable up to the maximum limit which would depend on the particular frequency.

A oneshot will not give constant duty cycle with varying frequency, unless you use a complicated feedback loop - which would have a response time issue.

 

[Hero999]

Looks better than Bertus's circuit to me. Same principle, fewer ICs (but a few more resistors).

Resistors are a heck of a lot cheaper than ICs.

It might better to use 82k for R5 and R6 and 100k for Pot1. Having both potentiometers the same value would be preferable because you don't have to stock more than one value for repairs.

A oneshot will not give constant duty cycle with varying frequency, unless you use a complicated feedback loop - which would have a response time issue.

That's true, the duty cycle will increase with frequency.

I was going to suggest varying the control voltage but I realised that it would alter the frequency.

 

[BrownOut]

Which is why (if you wanted to prevent 100% DC) you simply go to maximum frequency and dial in the R-C on the one shot to go to 95% duty cycle. Then, at lower frequencies, the maximum duty cycle is less than 95%.

The frequency would be wherever you set it, and the duty cycle would still be adjustable up to the maximum limit which would depend on the particular frequency.

Then, you're limited in lower frequencies as to how much duty cycle you can adjust. Not a good design, in my opinion.

 

[BrownOut]

What's wrong with my idea?

Did you even look at it?

Why have two ICs when you only need one?

The challenge was to use a timer for both frequency and duty cycle.

 

[Roff]

The challenge was to use a timer for both frequency and duty cycle.

I guess I missed that. What was the post number? And why did you carry on about oneshots, when that is clearly a separate circuit from a single timer IC?

 

[Hero999]

I can't remember anyone saying that it has to use a 555 timer. The original poster's idea used a 555, but that doesn't mean it's mandatory.

If using a 555 is a requirement, to get round it, just add a comparator.

If no additional active parts other than a 555 are aloud then, as far as I'm aware, it's not possible to produce independently controllable frequency and duty cycle.

Unless this is a college assignment, I don't see the point, just use an LM393 and be done with it. The LM393 is cheaper than a 555 anyway.

 

[BrownOut]

I said gold star if you can make your timer circuit both freq and duty cycle adjustable. Go back and look.

 

[Hero999]

I see, I obviously didn't notice.

I was helping the original poster, I don't have much interest in your challenge which is probably why I didn't pay any attention to it.

EDIT:
It looks like I did it anyway, since you didn't say anything about not using a comparator as well as the 555, do I get my gold star?

 

[BrownOut]

I'll give it a bronze star.

PS: had anyone investigated using the using the bias pin for frequency and threshold for duty cycle? I've given it some thought, but I just can't imagine you can get much frequency range from the bias pin.

 

[Roff]

I'll give it a bronze star.

PS: had anyone investigated using the using the bias pin for frequency and threshold for duty cycle? I've given it some thought, but I just can't imagine you can get much frequency range from the bias pin.

And changing the bias pin voltage will change the duty cycle.