LM555/NE555/SA555
Figure 7. Waveforms of Astable Operation
An astable timer operation is achieved by adding resistor R
B
to Figure 1 and configuring as shown on Figure 5. In the astable
operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multi
vibrator. When the timer output is high, its internal discharging Tr. turns off and the V
C1
increases by exponential
function with the time constant (R
A
+R
B
)*C.
When the V
C1
, or the threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal becomes high,
resetting the F/F and causing the timer output to become low. This in turn turns on the discharging Tr. and the C1 discharges
through the discharging channel formed by R
B
and the discharging Tr. When the V
C1
falls below Vcc/3, the comparator
output on the trigger terminal becomes high and the timer output becomes high again. The discharging Tr. turns off and the
V
C1
rises again.
In the above process, the section where the timer output is high is the time it takes for the V
C1
to rise from Vcc/3 to 2Vcc/3,
and the section where the timer output is low is the time it takes for the V
C1
to drop from 2Vcc/3 to Vcc/3. When timer output
is high, the equivalent circuit for charging capacitor C1 is as follows:
R
A
R
B
Vcc
C1
Vc1(0-)=Vcc/3
dv c1 V cc
–
V
(
0-
)
C
------------ = ------------------------------
-
-
1 dt
RA
+
RB
V
C1
(
0+
)
=
V
CC
(
1
)
(
2
)
⁄
3
t
-
– ------------------------------------
(
R A
+
R B
)C1
2
-
V C1
(
t
)
=
V CC
1
– --
e
3
(
3
)
Since the duration of the timer output high state(t
H
) is the amount of time it takes for the V
C1
(t) to reach 2Vcc/3,
6
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
