LM555/NE555/SA555
Application Information
Table 1 below is the basic operating table of 555 timer:
Table 1. Basic Operating Table
Threshold Voltage
Trigger Voltage
Discharging Tr.
Reset(PIN 4)
Output(PIN 3)
(V
th
)(PIN 6)
(V
tr
)(PIN 2)
(PIN 7)
Don't care
Don't care
Low
Low
ON
V
th
> 2Vcc / 3
High
Low
ON
V
th
> 2Vcc / 3
High
-
-
Vcc / 3 < V
th
< 2 Vcc / 3 Vcc / 3 < V
th
< 2 Vcc / 3
V
th
< Vcc / 3
High
High
OFF
V
th
< Vcc / 3
When the low signal input is applied to the reset terminal, the timer output remains low regardless of the threshold voltage or
the trigger voltage. Only when the high signal is applied to the reset terminal, the timer's output changes according 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 discharge Tr.
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 voltage, the timer's internal
discharge Tr. turns off, increasing the threshold voltage and driving the timer output again at high.
1. Monostable Operation
+Vcc
10
2
4
RESET
Trigger
8
Vcc
DISCH
R
A
=1
k
Ω
10
1
2
TRIG
THRES
Capacitance(uF)
10
0
6
C1
10
-1
3
OUT
GND
R
L
CONT
5
C2
10
-2
1
10
-3
10
-5
10
-4
R
10
-3
10
-2
10
-1
1M
10
0
10
M
10
1
A
A
7
10
0k
Ω
10
k
Ω
Ω
Ω
10
2
Time Delay(s)
Figure 1. Monoatable Circuit
Figure 2. Resistance and Capacitance vs.
Time delay(t
d
)
Figure 3. Waveforms of Monostable Operation
4
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
