NE555/SA555/SE555
APPLICATION INFORMATION
MONOSTABLE OPERATION
In the monostable mode, the timer functions as a
one-shot. Referring to figure 10 the external capaci-
tor is initially held discharged by a transistor inside
the timer.
Figure 10
V
CC
= 5 to 15V
Figure 11
t = 0.1 ms / div
INPUT = 2.0V/div
OUTPUT VOLTAGE = 5.0V/div
Reset
R1
4
Trigger
2
8
7
NE555
Output
6
C1
CAPACITOR VOLTAGE = 2.0V/div
3
1
5
Control Voltage
555-14.EPS
R1 = 9.1kΩ, C1 = 0.01µF, R
L
= 1kΩ
Figure 12
0.01
µ
F
The circuit triggers on a negative-going input signal
when the level reaches 1/3 Vcc. Once triggered, the
circuit remains in this state until the set time has
elapsed, even if it is triggered again during this in-
terval.The duration of the output HIGH stateis given
by t = 1.1 R
1
C
1
and is easily determined by
figure 12.
Notice that since the charge rate and the threshold
level of the comparator are both directly proportional
to supply voltage, the timing interval is independent
of supply. Applying a negativepulse simultaneously
to the reset terminal (pin 4) and the trigger terminal
(pin 2) during the timing cycle discharges the exter-
nal capacitor and causes the cycle to start over. The
timing cycle now starts on the positive edge of the
reset pulse. During the time the reset pulse in ap-
plied, the output is driven to its LOW state.
When a negativetrigger pulse is applied to pin 2, the
flip-flop is set, releasing the short circuit across the
external capacitor and driving the output HIGH. The
voltage across the capacitor increases exponen-
tially with the time constantτ = R
1
C
1
. When the volt-
age across the capacitor equals 2/3 V
cc
, the compa-
ratorresets the flip-flop which then discharge the ca-
pacitor rapidly and drivers the output to its LOW
state.
Figure 11 shows the actual waveforms generatedin
this mode of operation.
When Reset is not used, it should be tied high to
avoid any possibly or false triggering.
6/10
C
(µF)
10
1.0
0.1
0.01
0.001
10
µs
k
Ω
1
1=
k
Ω
R
10
k
Ω
00
Ω
1
1M
Ω
M
10
100
µs
1.0
ms
10
ms
100
ms
10
s
(t
d
)
ASTABLE OPERATION
When the circuit is connected as shown in figure 13
(pin 2 and 6 connected)it triggers itself and free runs
as a multivibrator. The external capacitor charges
through R
1
and R
2
and discharges through R
2
only.
Thus the duty cycle may be precisely set by the ratio
of these two resistors.
In the astable mode of operation, C
1
charges and
discharges between 1/3 V
cc
and 2/3 Vcc. As in the
triggeredmode, the chargeand discharge times and
therefore frequency are independent of the supply
voltage.
555-16.EPS
555-15.EPS
STMICROELECTRONICS [ STMICROELECTRONICS ]
