# How to increase 555 Timer Frequency

Discussion in 'General Electronics Discussion' started by Stuart, May 20, 2013.

1. ### Stuart

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Jan 24, 2013
I was wondering how to increase the frequency of a 555 timer? (pulse frequency)

I can't work out what to change; a resistor value or capacitor value?

Also, would it be possible to add a potentiometer or variable capacitor (whichever it is) to change the frequency on a circuit from the 555 timer and where would it go/what would it replace?

Here is a schematic of my 555 timer circuit if this helps to explain

Thanks

Stu

2. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
In your circuit if you want to maintain duty cycle you have to either change the value of C3 or change R3. R4 with like values or a ganged pot. By the way, what's the purpose of R2 (50uΩ)???

Chris

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
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Jan 21, 2010
I would suggest he is modelling the contact resistance.

"Why?" is another question.

4. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
Ha, in that case he might as well model the wire, component leads, battery contacts and PCB traces. Did I miss anything?

Chris

25,497
2,839
Jan 21, 2010

4,960
652
May 8, 2012
7. ### Stuart

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Jan 24, 2013
Thanks Chris that should help, and to answer your question R2 should actually be 100Ω I was just changing the values about to see what effect it had on my circuit that's why I put it in just to see what would happen haha should probably change it again before I forget.

Stu

Last edited: May 22, 2013
8. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
Then you're generating a near symmetrical waveform, IE Square Wave?

Chris

Edit: In that case you only need to vary R4

Last edited: May 21, 2013
9. ### BobK

7,682
1,688
Jan 5, 2010
The duty cycle of his circuit is 67%. The cap is charging through 2 68K resistors and discharging through 1 68K.

Bob

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
To change the frequency, while keeping the duty cycle the same, you can either change the capacitor (C3) (variable capacitors are generally not used however), OR the values of two resistors (R3 and R4). Changing 2 resistors at once is less than trivial when they are of different values.

There are other ways to design a 555 oscillator where a single resistor is used to both charge and discharge the cap (pin 3 -- the output -- is used to provide the charging/discharging voltage).

Is the duty cycle important (or do you actually want to change the duty cycle?). If a 50% duty cycle is OK, a combination of a single variable resistor and the use of pin 3 to charge/discharge will make the task of changing the frequency much easier.

11. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
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May 8, 2012
He said that was a misprint. R3 is only 100 Ohms.

Chris

12. ### BobK

7,682
1,688
Jan 5, 2010
I think he was actually talking about R2, which uOhm resistor being discussed at the time.

Bob

13. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
If he was then what you posted is true but he did specify R3. I guess we need conformation from Stu.

Chris

14. ### Stuart

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Jan 24, 2013
Yeah sorry I actually meant R2. R3 and R4 are both 68k

Stu

15. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I'm going to find you an beat you with a bat!

Then everything I said originally stands.
By the way R2 is not needed.

Chris

16. ### BobK

7,682
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Jan 5, 2010
Calm down, Chris,

You just have to learn how to not read, like I do.

Bob

17. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Bob, that would have worked for me before but as you can see below Stu has edited that post. All subsequent posts are now confusing and meaningless. Now I have to kill him.

Chris

18. ### BobK

7,682
1,688
Jan 5, 2010
Another forum I frequent only allows editing a post for a short time after it is posted. That would be a good policy here.

Bob

19. ### CDRIVEHauling 10' pipe on a Trek Shift3

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652
May 8, 2012
Here, here, I second that! These editing discussions are becoming far too frequent.

Chris

20. ### Stuart

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Jan 24, 2013
Haha oops, my bad. I did give a reason as to why I edited it though I just edited it so anyone reading could understand what I meant lol

Stu