NE555 Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

NE555

Single Timer

Fairchild Semiconductor 

LM555/NE555/SA555

Figure 1 illustrates a monostable circuit. In this mode, the timer generates a fixed pulse whenever the trigger voltage falls

below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low, the timer's

internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor C1

and setting the flip-flop output at the same time.

The voltage across the external capacitor C1, VC1 increases exponentially with the time constant t=RA*C and reaches 2Vcc/3

at td=1.1RA*C. Hence, capacitor C1 is charged through resistor RA. The greater the time constant RAC, the longer it takes

for the VC1 to reach 2Vcc/3. In other words, the time constant RAC controls the output pulse width.

When the applied voltage to the capacitor C1 reaches 2Vcc/3, the comparator on the trigger terminal resets the flip-flop,

turning the discharging Tr. on. At this time, C1 begins to discharge and the timer output converts to low.

In this way, the timer operating in the monostable repeats the above process. Figure 2 shows the time constant relationship

based on RA and C. Figure 3 shows the general waveforms during the monostable operation.

It must be noted that, for a normal operation, the trigger pulse voltage needs to maintain a minimum of Vcc/3 before the timer

output turns low. That is, although the output remains unaffected even if a different trigger pulse is applied while the output is

high, it may be affected and the waveform does not operate properly if the trigger pulse voltage at the end of the output pulse

remains at below Vcc/3. Figure 4 shows such a timer output abnormality.

Figure 4. Waveforms of Monostable Operation (abnormal)

2. Astable Operation

+Vcc

RA

4

8

RESET

Vcc

DISCH 7

2 TRIG

RB

THRES 6

3 OUT

C1

GND CONT 5

RL

1

C2

100

10

1

0.1

0.01

1E-3

100m

(RA+2RB)

1kΩ

10kΩ

100kΩ

1M

Ω

10M

Ω

1

10

100

1k

10k

100k

Fr equency(Hz)

Figure 5. Astable Circuit

Figure 6. Capacitance and Resistance vs. Frequency

5

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