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pulse generator

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samjesse

New Member
Hi

I built a pulse generator with some components for the local radio shack, 555 on a breadboard and ...
I tested it and it puts out a 50% duty cycle square wave. With a variable resistor I am able to control the frequency.
I want to build one where I control the duty cycle as well as the frequency.

how do I do this? any diagrams?

Thanks
 

Hero999

Banned
 

Hero999

Banned
Simple, have two potentiometers, one varies the on time and the other varies the off time.

Can you guess which pot. varies the on time and which varies the off time?
 

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samjesse

New Member
hummm, number 2 on the 555 is the trigger, so that would be the on time trigger. #4 is the rest so that would be the off time, no, well, I do not know.
 

Hero999

Banned
Sorry, I didn't label the schematic properly. Schematics should always be labelled so I've edited the schematic to add the labels.

When C2 charges via pin 3, Pot1 and D1, above 66% of the supply, pin 3 goes low, therefore Pot1 controls the on time.

When C2 discharges via pin 3, Pot2 and D1, below 33% of the supply, pin 3 goes high, therefore Pot2 controls the off time.

This circuit is for the 7555, you can use the 555 if you like but it won't be as good.
 
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Hero999

Banned
Yes, that was a typo.
 

BrownOut

Banned
Gold star if you can make the frequency and duty cycle adjustment independent. I've never figured out a way to do that.

EDIT: I should say that I actually have made them independent. However, adjusting one affects the range of the other. I was never able to come up with something that doesn't screw with the ranges.
 
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samjesse

New Member
Gold star if you can make the frequency and duty cycle adjustment independent. I've never figured out a way to do that.

EDIT: I should say that I actually have made them independent. However, adjusting one affects the range of the other. I was never able to come up with something that doesn't screw with the ranges.
How can they be made independent when one affects the range of the other? this is paradox.
 

BrownOut

Banned
How can they be made independent when one affects the range of the other? this is paradox.
I only said that I haven't been able to do it. You can adjust the frequency independent of duty-C, but then it affects the range. The quantities are independent, the ranges are not.
 
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samjesse

New Member
sorry, I misunderstood.
what is the different between the quantities and the ranges?

thx
 
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BrownOut

Banned
sorry, I misunderstood.
what is the different between the quantities and the ranges?

thx

Well for example, you can vary the frequency over a range, say from 10hz to 10khz. That's the range. But the particular frequency you're running at
at any particular time is the quantity. As an example, maybe you have a range of 10hz to 10khz, that's the frequencies that is possible from your design. But you've adjusted your circuit, to 5k hz. That's the quantity.
 

bountyhunter

Well-Known Member
Hi

I built a pulse generator with some components for the local radio shack, 555 on a breadboard and ...
I tested it and it puts out a 50% duty cycle square wave. With a variable resistor I am able to control the frequency.
I want to build one where I control the duty cycle as well as the frequency.

how do I do this? any diagrams?

Thanks
I have a circuit that does exactly what you want. I used two 555's so that the frequency and pulse width could be varied independently without affecting each other. The first 555 is a free running oscillator and the second is a one-shot. The oscillator triggers the one shot. You can vary the R-C timing components to get different frequencies and pulse widths.
 

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Hero999

Banned
Gold star if you can make the frequency and duty cycle adjustment independent. I've never figured out a way to do that.

EDIT: I should say that I actually have made them independent. However, adjusting one affects the range of the other. I was never able to come up with something that doesn't screw with the ranges.
I wouldn't use the 555, I'd use a dual comparator IC such as the LM393.
 

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BrownOut

Banned
Easy: 14 cents buys a second 555 timer....:D
The problem with that is as you adjust the frequency higher, then it becomes possible to adjust the one-shot period to a value longer than the period of the oscillator. Thus, the output frequency is affected by the DutyC adjustment.
 

bountyhunter

Well-Known Member
The problem with that is as you adjust the frequency higher, then it becomes possible to adjust the one-shot period to a value longer than the period of the oscillator. Thus, the output frequency is affected by the DutyC adjustment.
That's true, but the frequency is not changing. The duty cycle is simply going to 100%. The osc frequency is still the same.
 
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