• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.


Not open for further replies.


New Member
:D Hello to all,

It has come to notice that most problems encountered by the users of the famous 555 timer is the computation of TIME, RESISTANCE & CAPACITANCE. Usually we have the preconceived desired time value and capacitance value, but the resistance value is always a BIG question mark.

So i have come up with this prepared formulae to help out the members and would be members who would like to tinker with the 555 timer.

I. Comp. for unknown TIME w/a given R & C
T = 1.0986*R*C

II. Comp. for unknown R w/a given T & C
R = T / 1.0986*C

III. Comp. for unknown C w/a given T & R
C = T / 1.0986*R

Time total = Time on (Charging) + Time off (Discharging)
Time total = 0.693*(Ra + 2Rb)*C

I. Comp. for unknown Time off (Td) w/a given R & C
Td = 0.693*Rb*C

II. Comp. for unknown Rb w/a given Td & C
Rb = Td / (0.693*C)

III. Comp. for unknown C w/a given Rb & Td
C = Td / (0.693*Rb)

IV. Comp. for unknown Time on (Tc) w/a given (Ra + Rb) & C
Tc = 0.693*(Ra + Rb)*C
Tc = 0.693*(Ra + Td / 0.693*C)*C

V. Comp. for unknown Ra w/a given Rb, Tc & C
Ra = (Tc / 0.693*C) - Rb
Ra = (Tc / 0.693*C) - (Td / 0.693*C)

VI Comp. for unknown C w/a given Td, (Ra + Rb)
C = [Td / (Ra + Rb)] / 0.693

I hope those formulae would help. The constant figure of (0.693) is the natural log of 1/2 or 0.5 while the 1.0986 is the natural log of 1/3. These figures are the contribution of NETTRON 1000, thanks to him. :D :D :D


New Member
many thanks for those

I am just starting to play around with the 555 chip and this has made things a lot easier for me!!


New Member
reply to gjpolitt

:D hi gjpolitt,

am glad it was of help to you :D :D :wink:
Not open for further replies.

Latest threads

EE World Online Articles