NE555, SA555, SE555, SE555C
PRECISION TIMERS
SLFS022 – D1669, SEPTEMBER 1973—REVISED FEBRUARY 1992
APPLICATION INFORMATION
monostable operation
For monostable operation, any of these timers may be connected as shown in Figure 9. If the output is low,
application of a negative-going pulse to TRIG sets the flip-flop (Q goes low), drives the output high, and turns
off Q1. Capacitor C is then charged through R
A
until the voltage across the capacitor reaches the threshold
voltage of THRES input. If TRIG has returned to a high level, the output of the threshold comparator will reset
the flip-flop (Q goes high), drive the output low, and discharge C through Q1.
VCC
(5 V to 15 V)
RA
4
7
6
Input
2
5
CONT
8
VCC
RL
OUT
3
Output
RESET
DISCH
THRES
TRIG
GND
1
Voltage – 2 V/div
Time – 0.1 ms/div
Pin numbers shown are for the D, JG, and P packages.
Figure 9. Circuit for Monostable Operation
Monostable operation is initiated when TRIG
voltage falls below the trigger threshold. Once
initiated, the sequence ends only if TRIG is high at
the end of the timing interval. Because of the
threshold level and saturation voltage of Q1,
the output pulse duration is approximately
t
w
= 1.1 R
A
C. Figure 11 is a plot of the time
constant for various values of R
A
and C. The
threshold levels and charge rates are both directly
proportional to the supply voltage, V
CC.
The timing
interval is therefore independent of the supply
voltage, so long as the supply voltage is constant
during the time interval.
Applying a negative-going trigger pulse
simultaneously to RESET and TRIG during the
timing interval discharges C and re-initiates the
cycle, commencing on the positive edge of the
reset pulse. The output is held low as long as the
reset pulse is low. To prevent false triggering,
when RESET is not used, it should be connected
to V
CC
.
Figure 10. Typical Monostable Waveforms
10
RA = 10 MΩ
1
– Output Pulse Duration – s
RA = 1 MΩ
10–1
10–2
10–3
RA = 100 kΩ
10–4
RA = 10 kΩ
RA = 1 kΩ
10–5
0.001
0.01
0.1
1
10
100
tw
Figure 11. Output Pulse Duration vs Capacitance
POST OFFICE BOX 655303
•
DALLAS, TEXAS 75265
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RA = 9.1 kΩ
CL = 0.01
µF
RL = 1 kΩ
See Figure 9
Input Voltage
Output Voltage
Capacitor Voltage
C – Capacitance –
µF
9
Î
Î
TI [ TEXAS INSTRUMENTS ]
