CAPACITOR VOLTAGE = 2.0V/div

t = 0.1 ms / div

INPUT = 2.0V/div

OUTPUT VOLTAGE = 5.0V/div

R1 = 9.1k

Ω, C1 = 0.01µF, R = 1kΩ

L

Figure 11

Reset

Trigger

Output

R1

C1

Control Voltage

0.01

NE555

= 5 to 15V

V

CC

4

2

3

1

5

6

7

8

Figure 10

C

(

µF)

10

1.0

0.1

0.01

0.001

10

100

1.0

10

100

10

(t )

d

µs

µs

ms

ms

ms

s

10

M

Ω

1M

Ω

10

0k

Ω

10

kΩ

R

1=

1k

Ω

Figure 12

APPLICATION INFORMATION

MONOSTABLE OPERATION

In the monostable mode, the timer functions as a

one-shot. Referring to figure 10 the external capaci-

tor is initially held discharged by a transistor inside

the timer.

The circuit triggers on a negative-going input signal

when the level reaches 1/3 Vcc. Once triggered, the

circuit remains in this state until the set time has

elapsed, even if it is triggered again during this in-

terval.The duration of the output HIGH stateis given

figure 12.

Notice that since the charge rate and the threshold

level of the comparator are both directly proportional

to supply voltage, the timing interval is independent

of supply. Applying a negativepulse simultaneously

to the reset terminal (pin 4) and the trigger terminal

(pin 2) during the timing cycle discharges the exter-

nal capacitor and causes the cycle to start over. The

timing cycle now starts on the positive edge of the

reset pulse. During the time the reset pulse in ap-

plied, the output is driven to its LOW state.

When a negativetrigger pulse is applied to pin 2, the

flip-flop is set, releasing the short circuit across the

external capacitor and driving the output HIGH. The

voltage across the capacitor increases exponen-

tially with the time constant

rator resets the flip-flop which then discharge the ca-

pacitor rapidly and drivers the output to its LOW

state.

Figure 11 shows the actual waveforms generatedin

this mode of operation.

When Reset is not used, it should be tied high to

avoid any possibly or false triggering.

ASTABLE OPERATION

When the circuit is connected as shown in figure 13

(pin 2 and 6 connected)it triggers itself and free runs

as a multivibrator. The external capacitor charges

Thus the duty cycle may be precisely set by the ratio

of these two resistors.

triggeredmode, the charge and discharge times and

therefore frequency are independent of the supply

voltage.

NE555/SA555/SE555

6/10