Datasheet

CAPACITOR VOLTAGE = 2.0V/div
t = 0.1 ms / div
INPUT = 2.0V/div
OUTPUT VOLTAGE = 5.0V/div
R1 = 9.1kΩ, C1 = 0.01µF, R = 1kΩ
L
Figure 11
Reset
Trigger
Output
R1
C1
Control Voltage
0.01
µF
NE555
= 5 to 15V
V
CC
4
2
3
1
5
6
7
8
Figure 10
C
(µF)
10
1.0
0.1
0.01
0.001
10 100 1.0 10 100 10 (t )
d
µs µs
ms ms ms s
10M
Ω
1MΩ
10
0
k
Ω
10kΩ
R1= 1k
Ω
Figure 12
APPLICATION INFORMATION
MONOSTABLE OPERATION
In the monostable mode, the timer functions as a
one-shot.Referring to figure10 theexternal capaci-
tor is initially held discharged by a transistor inside
the timer.
The circuit triggers on a negative-going input signal
when thelevel reaches1/3 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 durationof theoutputHIGHstateisgiven
by t = 1.1 R
1
C
1
and is easily determined by
figure 12.
Notice that since the charge rate and the threshold
levelof thecomparatorarebothdirectlyproportional
to supply voltage, the timing intervalis 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-
nalcapacitorand causes thecycleto 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 negativetriggerpulse is appliedtopin 2, the
flip-flop is set, releasing the short circuit across the
externalcapacitor anddriving the outputHIGH. The
voltage across the capacitor increases exponen-
tiallywith the time constantτ =R
1
C
1
. When the volt-
ageacross thecapacitor equals2/3V
cc
, thecompa-
ratorresetstheflip-flopwhichthendischargethe ca-
pacitor rapidly and drivers the output to its LOW
state.
Figure 11 shows theactual waveforms generatedin
this mode of operation.
When Reset is not used, it should be tied high to
avoid any possibly or false triggering.
ASTABLE OPERATION
When the circuit is connectedas shown in figure 13
(pin 2and 6 connected)ittriggersitself andfreeruns
as a multivibrator. The external capacitor charges
through R
1
and R
2
and discharges through R
2
only.
Thus thedutycycle may bepreciselyset bythe ratio
of these two resistors.
In the astable mode of operation, C
1
charges and
dischargesbetween 1/3 V
cc
and 2/3 Vcc. As in the
triggeredmode,thechargeanddischargetimesand
therefore frequency are independentof the supply
voltage.
NE555/SA555/SE555
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