# 555 Timer Calculator Options

#### NFA Fabrication

Joined Aug 12, 2012
112
I used to have a 555 timer program that was a free download years ago, and it was amazing, but it stopped working with windows years ago. I was able to put in my desired times and voltage, and it would do the math on the component values needed. Every single one I find now wants me to put in values to see the time. It's kind of frustrating. Is there still one out there that works like that? I am trying to do an Astable 555 that is close to 5 seconds on/30 seconds off at ~12V.

#### SamR

Joined Mar 19, 2019
4,814

#### crutschow

Joined Mar 14, 2008
32,898
It has a reverse calculator which shows values for a given duty-cycle and astable frequency.
But for a duty-cycle of <50% you will need to invert the output signal or use a different circuit configuration, since the standard 555 astable configuration has a minimum duty cycle of slightly more than 50%.

Below is the LTspice simulation of an example circuit that does less than 50% duty-Cycle:
The diode allows the C2 timing capacitor discharge time to be longer than the charge.

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#### wayneh

Joined Sep 9, 2010
17,405
Read the PDF! LM555 Timer datasheet (Rev. D)

You will find both the equation for timing and Fig. 13 graphic of time delay.
I much prefer making my own calculator in Excel. Do it right once and you have it forever. It's much more flexible, because very few of the online tools allow, for instance, experimenting with the Control pin. Just be sure to document it as you go, so you can open it up in a few years and still be able to follow it.

#### crutschow

Joined Mar 14, 2008
32,898
very few of the online tools allow, for instance, experimenting with the Control pin.
How do you do that in Excel?

#### wayneh

Joined Sep 9, 2010
17,405
How do you do that in Excel?
A little rough but I think it's clear.

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#### crutschow

Joined Mar 14, 2008
32,898
A little rough but I think it's clear.
You mentioned the Control pin which is what I was particularly interested in, and I don't see that in the file.

#### wayneh

Joined Sep 9, 2010
17,405
You mentioned the Control pin which is what I was particularly interested in, and I don't see that in the file.
The "Vc" is the voltage applied to the control pin via R5. Does that clear it up?

#### crutschow

Joined Mar 14, 2008
32,898
The "Vc" is the voltage applied to the control pin via R5. Does that clear it up?
Not quite.
I see no "R5"
I do see a 2,200 to from Vc to the control pin.
What effect does that have on the calculations or circuit operation?

#### wayneh

Joined Sep 9, 2010
17,405
Not quite.
I see no "R5"
Oops, yes, I renamed it Rcontrol.
I do see a 2,200 to from Vc to the control pin.
That's it.
What effect does that have on the calculations or circuit operation?
Internal to the 555 are three 5K resistors in series between Vcc and ground. The voltages on the opposite ends of the center resistor determine the peak voltages on the timing cap. With no voltage applied to pin 5, those are 2/3 and 1/3 of Vcc. But pin 5 is connected to the top of the middle resistor and can be used to alter the voltages away from 2/3 and 1/3.

Columns C & D calculate those altered voltages as fractions of Vcc. Given those voltages, columns F & G calculate the time required to charge and discharge the timing capacitor. The frequency is then calculated from the combined charge/discharge time.

Rows 6, 7, & 8 give three cases for the applied voltage: None, or normal operation, then the two extremes of ground or Vcc.

#### crutschow

Joined Mar 14, 2008
32,898
Okay wayneh, thanks for the info.

#### NFA Fabrication

Joined Aug 12, 2012
112
It has a reverse calculator which shows values for a given duty-cycle and astable frequency.
But for a duty-cycle of <50% you will need to invert the output signal or use a different circuit configuration, since the standard 555 astable configuration has a minimum duty cycle of slightly more than 50%.

Below is the LTspice simulation of an example circuit that does less than 50% duty-Cycle:
The diode allows the C2 timing capacitor discharge time to be longer than the charge.

View attachment 235562
Thank you! This is exactly what I was hoping to find!

#### SamR

Joined Mar 19, 2019
4,814
The one @Delta prime listed is a bit much, as in very comprehensive, but a nice tool that I added to my busy desktop screen.