Datasheet
LM555 — Single Timer
© 2002 Fairchild Semiconductor Corporation www.fairchildsemi.com
LM555 Rev. 1.1.0 5
1. Monostable Operation
Figure 2 illustrates a monostable circuit. In this mode, the timer generates a fixed pulse whenever the trigger voltage
falls below V
CC
/3. When the trigger pulse voltage applied to the #2 pin falls below V
CC
/3 while the timer output is low,
the timer's internal flip-flop turns the discharging transistor off and causes the timer output to become high by charging
the external capacitor C1 and setting the flip-flop output at the same time.
The voltage across the external capacitor C1, V
C1
increases exponentially with the time constant t = R
A
*C and
reaches 2 V
CC
/3 at t
D
= 1.1 R
A
*C. Hence, capacitor C1 is charged through resistor R
A
. The greater the time constant
R
A
C, the longer it takes for the V
C1
to reach 2 V
CC
/3. In other words, the time constant R
A
C controls the output pulse
width.
When the applied voltage to the capacitor C1 reaches 2 V
CC
/3, the comparator on the trigger terminal resets the flip-
flop, turning the discharging transistor on. At this time, C1 begins to discharge and the timer output converts to low.
In this way, the timer operating in the monostable repeats the above process. Figure 3 shows the time constant rela-
tionship based on R
A
and C. Figure 4 shows the general waveforms during the monostable operation.
It must be noted that, for a normal operation, the trigger pulse voltage needs to maintain a minimum of V
CC
/3 before
the timer output turns low. That is, although the output remains unaffected even if a different trigger pulse is applied
while the output is high, it may be affected and the waveform does not operate properly if the trigger pulse voltage at
the end of the output pulse remains at below V
CC
/3. Figure 5 shows such a timer output abnormality.
2. Astable Operation
Figure 5. Waveforms of Monostable Operation
(abnormal)
Figure 6. A Stable Circuit Figure 7. Capacitance and Resistance vs. Frequency
1
5
6
7
8
4
2
3
RESET
Vcc
DISCH
THRES
CONT
GND
OUT
TRIG
+Vcc
R
A
C1
C2R
L
R
B
100m 1 10 100 1k 10k 100k
1E-3
0.01
0.1
1
10
100
10M
Ω
1M
Ω
100k
Ω
1
0k
Ω
1
k
Ω
(R
A
+2R
B
)
Capacitance(uF)
Frequency(Hz)