NA555, NE555, SA555, SE555
PRECISION TIMERS
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SLFS022F–SEPTEMBER 1973–REVISED JUNE 2006
APPLICATION INFORMATION (continued)
10
R
A
= 9.1 kΩ
R
A
= 10 MΩ
C = 0.01 µF
L
R
= 1 kΩ
L
1
See Figure 9
R
A
= 1 MΩ
−1
10
Input Voltage
−2
10
−3
−4
−5
10
10
10
R
= 100 kΩ
A
Output Voltage
R
= 10 kΩ
A
R
A
= 1 kΩ
Capacitor Voltage
Time − 0.1 ms/div
0.001
0.01
0.1
1
10
100
C − Capacitance − µF
Figure 10. Typical Monostable Waveforms
Figure 11. Output Pulse Duration vs Capacitance
Astable Operation
As shown in Figure 12, adding a second resistor, RB, 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
RA and RB and then discharges through RB only. Therefore, the duty cycle is controlled by the values of RA and
RB.
This astable connection results in capacitor C charging and discharging between the threshold-voltage level
(≈0.67 × VCC) and the trigger-voltage level (≈0.33 × VCC). As in the monostable circuit, charge and discharge
times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.
V
CC
R
R
= 5 kW
= 3 kW
R = 1 kW
See Figure 12
A
L
(5 V to 15 V)
B
C = 0.15 µF
0.01 µF
Open
(see Note A)
5
8
R
R
A
CONT
V
CC
R
L
4
7
RESET
DISCH
3
OUT
Output
6
2
B
t
THRES
TRIG
H
Output Voltage
t
L
GND
C
1
Pin numbers shown are for the D, JG, P, PS, and PW packages.
NOTE A: Decoupling CONT voltage to ground with a capacitor can
improve operation. This should be evaluated for individual
applications.
Capacitor Voltage
Time − 0.5 ms/div
Figure 12. Circuit for Astable Operation
Figure 13. Typical Astable Waveforms
10
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