I am trying to simulate a Monostable 555 timer that has an output pulse width of approximately 1 second. I happen to have a bunch of 47 uF caps, so I let C = 2*47 uF = 94 uF. We know that for monstable T = 1.1*R*C. Solving for R, R= T/(1.1C) = 1 sec/(1.1*C) = 9671 ohms. If I combine an 82k and 1.5k resistors in series I will have 9700 ohms, which is pretty close. Here is my screenshot, and I will attach the files.

 

If I combine an 82k and 1.5k resistors in series I will have 9700 ohms

I must be missing something here… possibly the decimal point placement

 

"If I combine an 82k and 1.5k resistors in series I will have 9700 ohms, ..."

Absolutely NOT!

Resistors in series, the result is greater than either component

\( 82\text{ KΩ}\;+\;1.5\text{ KΩ}\;=\;83.5\text{ KΩ} \)

Resistors in parallel, the result is less than either component

\( 82 \text{ KΩ} || 1.5\text{ KΩ}\;\approx\;1.47\text{ KΩ} \)

AFAIK, there is no combination two resistors that will produce an intermediate value.

 

82k in parallel with 1.5k isn't 9.7k. 82k in parallel with 11k is.

 

Obviously, TS meant 8.2k + 1.5k = 9.7kΩ

But I would not sweat over precision. Your 47μF capacitors alone are likely to be ±10% or ±20%.

 

Download the .asc sim file.

 

You need to use the "Skip initial operating point solution" command.

 

Sorry about the confusion. I meant 8.2k, which is in series with the 1.5k. I'm reading the rest of the replies now, and I will get back to you all.

 

Thanks Alec_t!!! I now get the desired result for the output. But the only thing i am wondering now is why my capacitor voltage slams to 0V after end the of the pulse. In theory caps, can't suddenly change voltage instantaneously. Yours looks right though.

 

i am wondering now is why my capacitor voltage slams to 0V after end the of the pulse. In theory caps, can't suddenly change voltage instantaneously.

The internal 555 DIS transistor to ground, turns on to discharge the capacitor for the next sequence.
It changes rapidly, but not instantly, based upon the capacitor size and the maximum transistor current to ground.
If you zoom in on the trace by using the mouse to draw a box around the fall, you will see the actual fall-time of the voltage.

On my sim I measure about a 1.5ms fall-time with a 94uF capacitor, showing a peak DIS current of about 550mA.

 

Thanks everyone, we can consider this one SOLVED!