# normally closed 555

Discussion in 'General Electronics Chat' started by devalvyas, Apr 30, 2009.

1. ### devalvyas Thread Starter Active Member

Nov 11, 2008
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0
hi,

i have come across this circuit of normally closed 555, but i am not sure on how to select the components for the pin 2.

i would like to have normally closed 555. my circuit should give me a 1 sec output pulse when i open the switch.

i know that i have to select the value of what value of 100k, 10k and 1 micro capacitor such that their time constant is much less than the control R and C at pin 6. but am confused about the actual values....should i select for 100k, 10k and 1 micro capacitor so that they dont control the timer.

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2. ### Wendy Moderator

Mar 24, 2008
21,463
2,959
The pulse generated by the signal conditioner (the components on pin 2) should be much less than the output pulse. I'd drop the capacitor value way down to achieve this. Other than that, it is very noncritical.

555 Monostable

3. ### devalvyas Thread Starter Active Member

Nov 11, 2008
80
0
i want to generate output pulse of around 1 sec. i am using the following values.

The timer resistor at pin 6 is around 9 K. The timer capacitor is 1000 micro farad. gives me output pulse time of 1.1*9000*0.0001=0.99 sec.

for conditioner circuit, i am using capacitor of 25 micro farad. now there are two resistors one 10k and another 100k connected to conditioner circuit. Now my confusing is regarding which resistor to consider while calculating input pulse duration?

Is the input pulse determined by 10K connected to ground, giving time constant of 1.1*10000*0.000025=0.275 sec?

OR

Is the input pulse determined by 100K connected to Vcc, giving time constant of 1.1*100000*0.000025=2.75 sec?

Or do i have to consider the divider circuit resistance created by 10k and 100k?

4. ### Wendy Moderator

Mar 24, 2008
21,463
2,959
Nope, the capacitor is fully charged, so the pulse is plenty big enough. Think 0.1µF for the conditioner cap though, you really don't need much there.