# Design Problem: 555 timer Constant DC, Variable Frequency

#### PhillipLawr2001

Joined May 24, 2019
10
View attachment 178170
So Imagine the Buzzer as a speaker. (couldn't find speaker symbol i think this helps my point.) This circuit i have tested and it works really well for a variable frequency and constant approx. 50% DC. The only problem is I'm having troubles trying to figure out the design for the RC network. I am trying to calculate the frequency based off a 10K pot with the trimmer halfway and a 110nF cap. The fixed resistor value I'm trying to find. I have the equation.
T = 2(ln(2) x C x (R)),
the R being the total resistance in the circuit and the whole equation multiplied by two because of Mark Time plus Space time added together. Just wanted to know if this is right. I believe this is the right way to calculate this and just curios if there something I am missing. I keep getting a frequency that is roughly 1261 Hz when i wanted a 440 Hz frequency when using this equation. I have looked at my circuit and their is nothing wrong with the build of it. So, right now i'm really confused and i am completely lost. if someone could help me that would be great.

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#### crutschow

Joined Mar 14, 2008
23,542
What values for Rtf, Rtv, and Ct give a value of 1261 Hz?

You need a resistor in series with the base of Q1, otherwise the base current is essentially the short-circuit output current of the 555.

#### PhillipLawr2001

Joined May 24, 2019
10
Yes, I agree I need a resistor in series with Q1. I have that in my breadboard but not on the schematic.

With this formula above i designed for a frequency of 440Hz, and i was using a 110nF cap and 10K pot as Rtv that is at approx 50% on the wiper portion, giving 5K Ohms. I also had a 10K fixed resistor as Rtf. given the equation i should get approx. 440Hz. The only problem is when i put that design on the breadboard with this exact schematic i was getting approx. 1261Hz instead of my 440Hz i have went over the breadboard for several days to see if there were any build error's. I cant find any so i think there is a design problem on my part.

hence the reason i gave my equation of T = 2(ln(2)(R)(C)) because of mark time plus space time added together to give me the total period. I think this equation is right to design this circuit, but the fact that im getting a frequency over two and half times my designed frequency is making it really weird for me.

#### crutschow

Joined Mar 14, 2008
23,542
My LTspice simulation below gives an oscillation frequency of 413Hz, so I suspect you have an error in your circuit wiring or component value.
Double-check both.

Joined Mar 10, 2018
3,631
The R and C have tolerances, in fact the the C, if you are using ceramics,
has a strong dependency on V depending on its dielectric that has to be
taken into consideration.

The 555 timer is not a high accuracy method of generating accurate time
characteristics.

And we have not discussed T & V and long term drift of components yet.

https://www.murata.com/en-us/support/faqs/products/capacitor/mlcc/char/0005

Regards, Dana.

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#### Alec_t

Joined Sep 17, 2013
10,470
You need a resistor in series with the base of Q1, otherwise the base current is essentially the short-circuit output current of the 555.
The base is driven by R1 and the discharge pin rather than the output pin. My sim shows negligible base current (apart from switching spikes of ~3mA max) even with no series base resistor.

#### crutschow

Joined Mar 14, 2008
23,542
The base is driven by R1
I was referring to his schematic in post #1.

#### Audioguru

Joined Dec 20, 2007
11,251
Since the output is a buzzing squarewave then it has many harmonic frequencies of the 440 Hz you want such as 1320Hz, 2200Hz, 3080hz and higher harmonics.

#### PhillipLawr2001

Joined May 24, 2019
10
just a response to the ceramic's idea. I am not using any ceramic capacitors. The only problem is the design or constructing the actual circuit. I'm just curios if someone could tell me if the design equation is right for this schematic and if so i have to look at the build of my circuit. A thought i had is that the Qchg transistor may have an effect to the RC network because of the voltage drop across it. Still doesn't explain though why it would change the designed frequency of 440Hz to a ridiculous frequency number like 1261Hz. The wired part is crutschow got around the same frequency as i designed for. So i think it might be a bad 555 timer chip or the circuti is built wrong.

any thoughts?

#### eetech00

Joined Jun 8, 2013
1,726
Hi

Try this circuit. Duty cycle is 50%.
"T" works out to approximately T=1.5*R*C, where R=R1+P1

Freq=444 Hz as shown below.

eT

#### crutschow

Joined Mar 14, 2008
23,542
Try this circuit.
That's the same as the TS's circuit except for the addition of D2, which I see no reason for.

#### PhillipLawr2001

Joined May 24, 2019
10
So, after looking at the circuit. What I built on the breadboard matched my schematic and i found the problem. The problem was a bad 555 timer IC and I had grabbed a different capacitor from the cabinet labeled 110nF but the capacitor was a different value. I fixed that capacitor part and tested the circuit then found out the timer IC itself was bad. So, I have a working circuit. The design was right and the schematic works beautiful.

Thanks to everyone for the help.

#### PhillipLawr2001

Joined May 24, 2019
10
The R and C have tolerances, in fact the the C, if you are using ceramics,
has a strong dependency on V depending on its dielectric that has to be
taken into consideration.

The 555 timer is not a high accuracy method of generating accurate time
characteristics.

And we have not discussed T & V and long term drift of components yet.

https://www.murata.com/en-us/support/faqs/products/capacitor/mlcc/char/0005

Regards, Dana.
Thanks for this, it was cool to learn but wasn't the issue. Thanks again

#### AnalogKid

Joined Aug 1, 2013
8,157
Content deleted - this is a homework question.

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#### crutschow

Joined Mar 14, 2008
23,542
The output frequency is just a little bit lower than the standard equation predicts because the 555 output stage does not swing all the way up to Vcc.
A standard 555 will also not have an exact 50% duty-cycle with that circuit, since that lower value of the high output voltage creates a different voltage difference for the charge and discharge ramps.

#### ElectricSpidey

Joined Dec 2, 2017
739
Did you mean that configuration can’t produce a true 50/50 duty cycle due to the asymmetrical design of the output stage?

#### Audioguru

Joined Dec 20, 2007
11,251
Did you mean that configuration can’t produce a true 50/50 duty cycle due to the asymmetrical design of the output stage?
Yes, the output stage is like old fashioned asymmetrical TTL, not symmetrical like Cmos.
There is a more modern Cmos 555 available with a symmetrical output: LMC555, TLC555 and ICM7555 but the maximum output current is less than an old 555.

#### AnalogKid

Joined Aug 1, 2013
8,157
Content deleted - this is a homework question.

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#### PhillipLawr2001

Joined May 24, 2019
10
@AnalogKid I will definenitly try this thanks. It's a lot simpler than my first schematic. Plus this is a project for school so i have to stick with the main premise of the course. Also i was aware that my first schematic wasn't a true 50% DC.

#### WBahn

Joined Mar 31, 2012
24,854
MOD NOTE: Moved to Homework Help.