Datasheet

NE555, NE555Y, SA555, SE555, SE555C
PRECISION TIMERS
SLFS022A – SEPTEMBER 1973 – REVISED SEPTEMBER 2000
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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 charges through
R
A
and R
B
and then discharges through R
B
only. Therefore, the duty cycle is controlled 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.
GND
OUT
V
CC
CONT
RESET
DISCH
THRES
TRIG
C
R
B
R
A
Output
R
L
0.01
µF
V
CC
(5 V to 15 V)
(see Note A)
NOTE A: Decoupling CONT voltage to ground with a
capacitor can improve operation. This should be
evaluated for individual applications.
Open
58
4
7
6
2
3
1
Pin numbers shown are for the D, JG, and P packages.
Figure 12. Circuit for Astable Operation
Voltage – 1 V/div
Time – 0.5 ms/div
t
H
Capacitor Voltage
Output Voltage
t
L
R
A
= 5 kΩ R
L
= 1 kΩ
R
B
= 3 k
Ω See Figure 12
C = 0.15 µF
Figure 13. Typical Astable Waveforms