How to increase 555 Timer Frequency

[Stuart]

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

 

[CDRIVE]

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

 

[(*steve*)]

By the way, what's the purpose of R2 (50uΩ)???

I would suggest he is modelling the contact resistance.

"Why?" is another question.

 

[CDRIVE]

I would suggest he is modelling the contact resistance.

"Why?" is another question.

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

Chris

 

[(*steve*)]

Did I miss anything?

Solder joints.

These can be very important even if you have a Kelvin connection on the PCB to a sense resistor.

http://www.analog.com/library/analogDialogue/archives/46-06/shunt_resistors.html

 

[CDRIVE]

Solder joints.

These can be very important even if you have a Kelvin connection on the PCB to a sense resistor.

http://www.analog.com/library/analogDialogue/archives/46-06/shunt_resistors.html

Don't think Stuart needs that here but if he does a high current measuring project this will be very useful.

Chris

 

[Stuart]

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

 

[CDRIVE]

Thanks Chris that should help, and to answer your question R3 should actually be 100Ω
Stu

Then you're generating a near symmetrical waveform, IE Square Wave?

Chris

Edit: In that case you only need to vary R4

 

[BobK]

The duty cycle of his circuit is 67%. The cap is charging through 2 68K resistors and discharging through 1 68K.

Bob

 

[(*steve*)]

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.

 

[CDRIVE]

The duty cycle of his circuit is 67%. The cap is charging through 2 68K resistors and discharging through 1 68K.

Bob

He said that was a misprint. R3 is only 100 Ohms.

Chris

 

[BobK]

I think he was actually talking about R2, which uOhm resistor being discussed at the time.

Bob

 

[CDRIVE]

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

Chris

 

[Stuart]

Yeah sorry I actually meant R2. R3 and R4 are both 68k

Stu

 

[CDRIVE]

Yeah sorry I actually meant R2. R3 and R4 are both 68k

Stu

I'm going to find you an beat you with a bat!

Then everything I said originally stands.

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

By the way R2 is not needed.

Chris

 

[BobK]

Calm down, Chris,

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

Bob

 

[CDRIVE]

Calm down, Chris,

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

Bob

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

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

 

[BobK]

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

 

[CDRIVE]

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

Chris

 

[Stuart]

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