555 timer frequency changing

[ants]

I have a 555 timer circuit that I made up. It has a frequency of 100KHz. When I attach it to my amp through a potentiometer the frequency seems to drop to 80KHz. Is this right? I wasn't expecting that to happen.

 

[IanP]

That may depend on the configuration ..
If the output pin [3] is somehow used to generate a square wave (e.g. 50% duty cycle) then obviously connecting "something" to this pin may vary the frequency .. see picture below ..
If this pin [3] is used only as the output - the frequency should be independent ..

IanP

 

[ants]

It seems I've lost abut 1Khz because of that, i compared the 555 connected to the amp and then not.

But I've lost 14Khz taking R1 and R2 off a breadboard and putting them on the rest of the board. I made up the 555 and left those two off to adjust, then put them on last. How can switching the same components cause such a big change in frequency? Itys confusing. I will have to take them off the board again, then add a pot to R2 to give myself some leeway.

 

[IanP]

The capacitance between any two adjacent tracks on any breadboard may easily rich 33-47pF, so be careful when you try to fine-tune circuit on a breadboard and then install them on a veroboard or the target PCB ..
In comparison, a pcb or Veroboard would have about 1-2-3pF ..

IanP

 

[ants]

I have it working at target frequency, the duty cycle isn't as close to 50% as I'd like but its not worth worrying about. The capacitance value is 100pF but on the veroboard it is measured at 330pF. It is that which has been causing the problem. I put a potentiometer in place of R2 and adjusted till i found the right frequency and have now measured everything.

I'll bare in mind what a run around that is for future reference!

 

[IanP]

I have it working at target frequency, the duty cycle isn't as close to 50% as I'd like but its not worth worrying about.

The way you can have exactly 50% duty cycle at all times would be to generate double the frequency and then divide it by two using a flip-flop ..

The capacitance value is 100pF but on the veroboard it is measured at 330pF

Hm … it looks like the veroboard must have been manufactured by Kuku-Maluku from Limpopo .. Funny as it sounds, the difference between 100pF cap and the measured value of 330pF is just to big .. Something else must have increased the capacitance, but what?

IanP

 

[ants]

I've heard of frequency splitting and may come back to that. I have a 10uF cap on the output, this allows me to get a bipolar signal from the circuit, perhaps that is having some kind of effect. If I do the calcs for frequency and the measured R1, R2 then it implies C is 170pF, that is a bit closer. I think I would have to take components back off to find out exactly what is going on with that.

I have just more or less completed my project, a piezo driver, it has taken a couple of months, maybe 3. I built an audio amp with high GBP first off, then had to find the right transformer, then for test build a dual power supply, I will use batteries for the final portable version, then got the 555 working. Next thing is to make up a spare in case this one decides to wilt, and finally put it all in a nice box so it looks better.

 

[IanP]

Sounds like an ambitious project ..
And don’t worry about some small discrepancies here and there ..

I wish you all the best ..
IanP

 

[ants]

Was a bit ambitious, especially as it was my first and only project. I wish making the piezo was as quick, for that I have to rely on industrial companies, lead times can be 3 months and what they send may not work, so I have to start all over again. We'll see. Thanks.