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