KA555

4

Application Information

Table1 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 voltage or

the trigger voltage. Only when the high signal is applied to the reset terminal, 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. Monos Table Operation

Table 1. Basic Operating Table

Threshold Voltage

Trigger Voltage

Reset(Pin4)

Output(Pin3)

Discharging Tr.

(Pin7)

Don't care

Don't care

Low

Low

ON

High

Low

ON

High

-

-

High

High

OFF

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)

Figure 1. Monoatable Circuit

Figure 2. Resistance and Capacitance vs.

Figure 3. Waveforms of Monostable Operation

1

5

6

7

8

4

2

3

RESET

Vcc

DISCH

THRES

CONT

GND

OUT

TRIG

+Vcc

C1

C2

Trigger