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# 555 TIMER FORMULAE

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#### mozikluv

##### New Member
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.

MONOSTABLE MODE:
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

ASTABLE MODE:
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.

#### gjpollitt

##### 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!!

#### mozikluv

##### New Member
reply to gjpolitt

hi gjpolitt,

am glad it was of help to you :wink:

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