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NE555M

Precision timer

Wing Shing International Group 

Applications Information

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot

(Figure 1). The external capacitor is initially held discharged

by a transistor inside the timer. Upon application of a nega-

tive trigger pulse of less than 1/3 VCC to pin 2, the flip-flop is

set which both releases the short circuit across the capacitor

and drives the output high.

NE555

NOTE: In monostable operation, the trigger should be driven

high before the end of timing cycle.

FIGURE 1. Monostable

The voltage across the capacitor then increases exponen-

tially for a period of t = 1.1 RA C, at the end of which time the

voltage equals 2/3 VCC. The comparator then resets the

flip-flop which in turn discharges the capacitor and drives the

output to its low state. Figure 2 shows the waveforms gener-

ated in this mode of operation. Since the charge and the

threshold level of the comparator are both directly propor-

tional to supply voltage, the timing internal is independent of

supply.

FIGURE 3. Time Delay

ASTABLE OPERATION

If the circuit is connected as shown in Figure 4 (pins 2 and 6

connected) it will trigger itself and free run as a multivibrator.

The external capacitor charges through RA + RB and dis-

charges through RB. Thus the duty cycle may be precisely

set by the ratio of these two resistors.

VCC = 5V

TIME = 0.1 ms/DIV.

RA = 9.1kΩ

C = 0.01µF

Top Trace: Input 5V/Div.

Middle Trace: Output 5V/Div.

Bottom Trace: Capacitor Voltage 2V/Div.

FIGURE 2. Monostable Waveforms

During the timing cycle when the output is high, the further

application of a trigger pulse will not effect the circuit so long

as the trigger input is returned high at least 10µs before the

end of the timing interval. However the circuit can be reset

during this time by the application of a negative pulse to the

reset terminal (pin 4). The output will then remain in the low

state until a trigger pulse is again applied.

When the reset function is not in use, it is recommended that

it be connected to VCC to avoid any possibility of false trig-

gering.

Figure 3 is a nomograph for easy determination of R, C val-

ues for various time delays.

FIGURE 4. Astable

In this mode of operation, the capacitor charges and dis-

charges between 1/3 VCC and 2/3 VCC. As in the triggered

mode, the charge and discharge times, and therefore the fre-

quency are independent of the supply voltage.

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