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.