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555 Astable
timer
With the external net (wich includes Ra,Rb and C) , it is
possible to use the 555 as an astable (free running)oscillator.
The capacitor is connected to the non inverting input of
the first comparator and to the inverting input of the second
one.
Suppose that Vout is high .
Q is high and Qn is low.
Qn=
The BJT transistor is in off state (interdiction). So, because
of the comparators don't drain current,the capacitor C can
charge itself with an exponential law and with a time costant
(Ra+Rb)C .
Vc is rising to Vcc but when it reaches the V1 potential
(V1=2/3Vcc),the output of the first comparator goes down and
the reset of the bistable circuit force Q to low and Qn to
high. In this way , the transistor is in the On state and
the capacitor discharge with a time costant of Rb C . When
Vc reaches the V2 voltage (1/3 Vcc) , the output of the second
comparator toggle to the high state and Q is set to high while
Qn to low.
It is possible to calculate T1 with the following
equations (capacitor charge equation):
Because of Vc(t1)=Vc(T1)=V1=2/3Vcc ,
T1=(Ra+Rb)C ln 2
In the same way , with the discharge law,
T2=Rb C ln2
T1+T2=T=1/f
Examples:
With:
Ra=Rb=4.7 K ohms
C=100uF
Vcc=+5V
you get a TTL output of 1Hz .
With:
Ra=Rb=330 ohms
C=0.1uF
Vcc=+5V
you get a ~10Khz TTL output.
Jef Verive has written a software that let you
calculate the values of Ra,Rb and C in order to get the right
T.
Last know working url is:
http://www.oocities.org/jverive/
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