555 Astable Oscillator


                    +-+--------------Vcc
                    | |
                    | /
                    | \ Ra
            ______  | /
         0-[1    8]-+ |    Rb
       +---[      ]---+--/\/\-+
  o----|---[3     ]-----------+
       | +-[______]           |
       | |                    |
       +-|--------------------+
         |                    |
         Vcc                 ---
                             --- Ct
                              |
                              +----- 0V


Output:
                           T2
...-------+   +-----------+   +--------...
          |   |           |   |
          +---+           +---+
                   T1

T1=0.7*(Ra+Rb)*Ct  (output pin3 is 1)

T2=0.7*Rb*Ct

Total period = T = T1 + T2

for 50hz clock T=1/50 = 0.02 seconds

the constant 0.7 is acually ln2 (natural log of 2) which is 0.6931...

Hence T = 0.7*(Ra+2*Rb)*Ct


For the case of T1=T2 we can omit these resistors, and put another
between pin2 and pin3

in this case the T becomes 1.4*Rt*Ct

all units are SI, that is:

R in ohms
C in farads (not microfarads)
T in seconds
F in hertz

Maybe in my notes I used uF instead of F

Now, for T=0.02  using T=2*ln(2)*R*C ~= T=1.4*R*C

RC=0.01443

for 3300 ohm C becomes 0.01443/3300 = 4.37*10E-6 ~= 4.4uF

and other values for standard C values are:

uF     ohm
-----  -------
C=1    R=14426 *
C=1.5  R=9617
C=2.2  R=6557
C=3.3  R=4371
C=4.7  R=3069
C=6.8  R=2121
C=10   R=1442 *

note: marked ones can be used with %1 ranges resistors 14.3kohm and 1.43kohm


Anyway, maybe building the first version should fix the unknown problem.

in this case, we use T1=T2=0.01 seconds:

0.01=0.7*Rb*Ct

take Ct as 4.7uF for example:

Rb=0.01*1000000/(0.7*4.7)
Rb=3Kohm

and using T1=0.7*(Ra+Rb)*Ct

Ra=10000/(0.7*4.7)-3000
Ra=40ohm

actually, if we equate the equations we see that Ra goes to zero as
T1 becomes closer to T2.
But we cannot take Ra as 0 in this circuit. Because 555 works by
comparing the voltages.

Ehm, this is not a perfect solution. But it may suit the needs.


Standard resistor values are here:
http://www.rfcafe.com/references/electrical/resistor_values.htm




My regards,

Ilker Ficicilar

ilker(@)ekitap.gen.tr
ilker(@)ficicilar.name.tr
http://cbm.ficicilar.name.tr/

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