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# How do I calculate the high time and low time of a 555 timer circuit? R1= 1K Ohms R2= 68k Ohms C= 2.2 pF?

R1= 1K Ohms

R2= 68k Ohms

C= 2.2 pF

Update:

Right, I know the equations but I cant figure out how to calculate Ohms vs Pico Farads... Forgot to add that in.

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- billrussell42Lv 71 decade agoFavorite Answer
2 pF is much too low a value for this application. The timing cap should be at least 1000 pf for any accuracy. It can be as low as 200 pF.

in the one-shot mode, t = 1.1 RC where R is the resistor between pin 7 and VCC, and C is the capacitor from pin 7 to ground.

In the astable or free running mode, there are two times, high and low.

the reference has more info.

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Source(s): http://en.wikipedia.org/wiki/555_timer_IC - Rich ZLv 71 decade ago
Here is a site that helps you enter component values for it to calculate with (and lets you run a simulation)

Source(s): a picoFarad is 10^-12 Farads (a millionth of a microFarad).

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