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
(V )(PIN 2)
tr
Discharging Tr.
(PIN 7)
Reset(PIN 4)
Output(PIN 3)
(V )(PIN 6)
th
Don't care
Don't care
Low
High
High
High
Low
Low
-
ON
ON
-
V
> 2Vcc / 3
V
> 2Vcc / 3
th
th
Vcc / 3 < V < 2 Vcc / 3 Vcc / 3 < V < 2 Vcc / 3
th
th
V
< Vcc / 3
V
< Vcc / 3
th
High
OFF
th
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
102
R
A
4
8
101
100
RESET
Vcc
Trigger
7
6
DISCH
TRIG
OUT
2
3
THRES
CONT
10-1
10-2
10-3
C1
5
GND
R
L
C2
1
10-5
10-4
10-3
10-2
10-1
100
101
102
Time Delay(s)
Figure 2. Resistance and Capacitance vs.
Time delay(t )
Figure 1. Monoatable Circuit
d
Figure 3. Waveforms of Monostable Operation
4
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
