Datasheet

LM555 — Single Timer
© 2002 Fairchild Semiconductor Corporation www.fairchildsemi.com
LM555 Rev. 1.1.0 4
Application Information
Table 1 below is the basic operating table of 555 timer.
When the low signal input is applied to the reset terminal, the timer output remains low regardless of the threshold volt-
age or the trigger voltage. Only when the high signal is applied to the reset terminal, the timer's output changes accord-
ing to threshold voltage and trigger voltage.
When the threshold voltage exceeds 2/3 of the supply voltage while the timer output is high, the timer's internal dis-
charge transistor turns on, lowering the threshold voltage to below 1/3 of the supply voltage. During this time, the timer
output is maintained low. Later, if a low signal is applied to the trigger voltage so that it becomes 1/3 of the supply volt-
age, the timer's internal discharge transistor turns off, increasing the threshold voltage and driving the timer output
again at high.
1. Monostable Operation
Table 1. Basic Operating Table
Reset
(PIN 4)
V
TR
(PIN 2)
V
TH
(PIN 6)
Output
(PIN 3)
Discharging
Transistor
(PIN 7)
Low X X Low ON
High < 1/3 V
CC
XHighOFF
High > 1/3 V
CC
> 2/3 V
CC
Low ON
High > 1/3 V
CC
< 2/3 V
CC
Previous State
Figure2. Monostable Circuit
Figure 3. Resistance and Capacitance vs.
Time Delay (t
D
)
Figure 4. Waveforms of Monostable Operation
1
5
6
7
8
4
2
3
RESET
Vcc
DISCH
THRES
CONT
GND
OUT
TRIG
+Vcc
R
A
C1
C2R
L
Trigger
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
M
Ω
1M
Ω
1
0k
Ω
100k
Ω
R
A
=
1k
Ω
Capacitance(uF)
Time Delay(s)