© 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)

(PIN 2)

(PIN 6)

Output

(PIN 3)

Discharging

Transistor

(PIN 7)

Low

X

X

Low

ON

High

XHigh

OFF

High

Low

ON

High

Previous State

Figure2. Monostable Circuit

Figure 3. Resistance and Capacitance vs.

Figure 4. Waveforms of Monostable Operation

1

5

6

7

8

4

2

3

RESET

Vcc

DISCH

THRES

CONT

GND

OUT

TRIG

+Vcc

C1

C2

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

Time Delay(s)