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


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

2 Answers

  • 1 decade ago
    Favorite 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.


  • Rich Z
    Lv 7
    1 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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