Found some freeware that has quite a few calculators for math the challenged.

Stuff like potential dividers, 555 timer timing, ETC, ETC. At least have a quick look.


http://www.electronics2000.co.uk/download.php#assistant

 

Found some freeware that has quite a few calculators for math the challenged.

Stuff like potential dividers, 555 timer timing, ETC, ETC. At least have a quick look.


http://www.electronics2000.co.uk/download.php#assistant

I like it, but the results for the 555 astable calculations don't agree with the formula I have been taught. For example, with a 555 without a diode, and with C1 = 100uF, R1 = 6800, and R2 = 910, the calculator gives a frequency of 1.657Hz and a duty cycle of 89%. My formula gives a frequency of .9649Hz and a duty cycle of 53.14%. Am I wrong?

 

I like it, but the results for the 555 astable calculations don't agree with the formula I have been taught. For example, with a 555 without a diode, and with C1 = 100uF, R1 = 6800, and R2 = 910, the calculator gives a frequency of 1.657Hz and a duty cycle of 89%. My formula gives a frequency of .9649Hz and a duty cycle of 53.14%. Am I wrong?

If R1=RA (in the datasheet), and R2=RB, Then the frequency is 0.992Hz. If the duty cycle is defined as T(hi)/Period, then the National datasheet is wrong. It gives the duty cycle if it is defined as T(lo)/Period.
With Duty cycle=T(hi)/Period, then the duty cycle is (R1+R2)/(R1+2R2)=53.14%.
I think the downloadable calculator sets R1=RB (in the datasheet), and R2=RA. If this is done, I get F=1.6705Hz and 89% duty cycle.

 

If R1=RA (in the datasheet), and R2=RB, Then the frequency is 0.992Hz. If the duty cycle is defined as T(hi)/Period, then the National datasheet is wrong. It gives the duty cycle if it is defined as T(lo)/Period.
With Duty cycle=T(hi)/Period, then the duty cycle is (R1+R2)/(R1+2R2)=53.14%.
I think the downloadable calculator sets R1=RB (in the datasheet), and R2=RA. If this is done, I get F=1.6705Hz and 89% duty cycle.

Thanks Ron. So, if we go by the National datasheet, my formula for the duty cycle is wrong, but my formula for the frequency is correct?

Attached is the pertinent info from the ST datasheet. Using their figure and formula with R1 = 6800, R2 = 910, C1 = 100uF, I get 1.6705Hz for f. Their formula for duty cycle gives 10.56%, which means they are defining duty cycle as T(lo)/Period.

I am still confused. Maybe it will make sense in the morning.

 

Thanks Ron. So, if we go by the National datasheet, my formula for the duty cycle is wrong, but my formula for the frequency is correct?

Attached is the pertinent info from the ST datasheet. Using their figure and formula with R1 = 6800, R2 = 910, C1 = 100uF, I get 1.6705Hz for f. Their formula for duty cycle gives 10.56%, which means they are defining duty cycle as T(lo)/Period.

I am still confused. Maybe it will make sense in the morning.

I think we need a labeled schematic to refer to.
I'm just as confused as you are. Maybe my previous post was wrong. I'll have to revisit it in the morning.

 

I think we need a labeled schematic to refer to.
I'm just as confused as you are. Maybe my previous post was wrong. I'll have to revisit it in the morning.

A labled and annotated schematic is attached for your review. I am off to bed now.

 

Found some freeware that has quite a few calculators for math the challenged.

So that the math challenged remain math challenged. Or, more likely in this case, so that people that don't understand voltage dividers can go on not understanding voltage dividers.

Tools are great when we use them to make doing something we comprehend easier and/or less error prone. But we (generic "we" -- applies to most, but not all, people) too quickly are willing to use them as substitutes for any semblence of comprehension on our part.

 

@WBahn, while I agree with you about the tools over learning, at 65 years old and not having had to do algebra since high school, I need all the help I can get.

And I do understand the principles behind the voltage divider and 555, just makes it easier to use a calculator.

Spent my life as a tool and die maker and the most math used was trig to figure angles and dimensions to make the parts to size. But even then we used a calculator to speed things up. Time was money.

 

Oh, to be sure I wasn't talking about you (or anyone) in particular. Just a general observation that flowed from a certain remark you made.

 

I still don't fully understand this, but I am partially satisfied that the formula for frequency of a 555 astable is

f = 1.44 / ((RA + 2RB) x C)

At least, that's what the Fairchild, National, TI, and ST datasheets say. The Phillips and Unitech datasheets use 1.49 instead of 1.44 in the formula.

According to the help screen on the electronics calculator referenced in the original post, the formula they use is

f = 1.4 / ((R1 + 2R2) x C1)

So, there is the reason for one discrepancy. In addition, the same help screen says the formula they use for total time is

T = 0.7 x (R1 + 2R2) x C1

Most other sources use 0.693 instead of of 0.7, so there is another reason for discrepancy in results.

Finally, most sources agree that duty cycle percentage given refers to the percentage of time that the output of the 555 is low (unless otherwise stated.) That seems backwards to me, but maybe that's just me.

Probably no one cares about this but me, but the same pedantic nature that causes me to delve into such minutia causes me to post some sort of a conclusion.

And I still like the calculator because I am definitely one of the "math challenged."

 

I never go by the formulas. If I do use them, they are just a guideline for me.
You may use 5% or 1% resistors but what's the point. Most of the capacitors are 10% and 20% anyway. Hence I wouldn't worry too much about the difference between 1.4, 1.44 or 1.49.

1/1.4 is pretty close to 0.7
0.7 or 0.693, same thing.

The point is, don't get carried away with precision. I plug into the circuit components with values in the ball park then I adjust to get the required behaviour.

Duty cycle is high/period or low/period, same difference. One is one minus the other.
Don't stress yourself over it.