NE555, SA555, SE555, SE555C
PRECISION TIMERS
SLFS022 – D1669, SEPTEMBER 1973—REVISED FEBRUARY 1992
APPLICATION INFORMATION
astable operation
As shown in Figure 12, adding a second resistor, R
B,
to the circuit of Figure 9 and connecting the trigger input
to the threshold input causes the timer to self-trigger and run as a multivibrator. The capacitor C will charge
through R
A
and R
B
and then discharge through R
B
only. The duty cycle may be controlled, therefore, by the
values of R
A
and R
B.
This astable connection results in capacitor C charging and discharging between the threshold-voltage level
(≈ 0.67•V
CC
) and the trigger-voltage level (≈ 0.33•V
CC
). As in the monostable circuit, charge and discharge
times (and therefore the frequency and duty cycle) are independent of the supply voltage.
0.01
µ
F
8
VCC
RL
3
OUT
THRES
TRIG
GND
C
1
Output
Voltage – 1 V/div
Open
(see Note A) 5
CONT
RA
4
7
6
2
RESET
DISCH
RB
tH
tL
Output Voltage
Pin numbrs shown are for the D, JG, and P packages.
NOTE A: Decoupling CONT voltage to ground with a
capacitor may improve operation. This should be
evaluated for individual applications.
Figure 12. Circuit for Astable Operation
10
POST OFFICE BOX 655303
•
DALLAS, TEXAS 75265
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RL = 1 kΩ
See Figure 12
Capacitor Voltage
Time – 0.5 ms/div
VCC
(5 V to 15 V)
RA = 5 k
Ω
RB = 3 k
Ω
C = 0.15
µF
Î
Î
Figure 13. Typical Astable Waveforms
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