# [SOLVED] Monostable 555 Timer and Switch Woes

#### WARDEVIL_UFO

Joined Nov 16, 2010
15
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.

#### djsfantasi

Joined Apr 11, 2010
9,160
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

#### Papabravo

Joined Feb 24, 2006
21,225
"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.

#### dl324

Joined Mar 30, 2015
16,900
82k in parallel with 1.5k isn't 9.7k. 82k in parallel with 11k is.

#### MrChips

Joined Oct 2, 2009
30,792
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%.

#### crutschow

Joined Mar 14, 2008
34,405

#### Alec_t

Joined Sep 17, 2013
14,312
You need to use the "Skip initial operating point solution" command.

#### WARDEVIL_UFO

Joined Nov 16, 2010
15
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.

#### WARDEVIL_UFO

Joined Nov 16, 2010
15
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.

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#### crutschow

Joined Mar 14, 2008
34,405
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.