NE555/SA555/NA555

Document number: DS35112 Rev. 5 - 2

7 of 14

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February 2021

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NE555

Typical Applications Characteristics

Monostable Operation

For monostable operation, any of the ‘555 timers can be connected as shown in Figure 1. If the output is low, application of a negative-going

pulse to the trigger (TRIG) sets the internal flip-flop and drives the output high. Capacitor C is then charged through RA until the voltage

across the capacitor reaches the threshold voltage of the threshold (THRES) input. If TRIG has returned to a high level, the output of the

threshold comparator resets the internal flip-flop, drives the output low, and discharges C.

Fig. 1 Monostable operation

Monostable operation is initiated when TRIG voltage falls below the trigger threshold. Once initiated, the sequence ends only if TRIG is high

for at least 10μs before the end of the timing interval. When the trigger is grounded, the comparator storage time can be as long as 10μs,

which limits the minimum monostable pulse width to 10μs. Because of the threshold level and saturation voltage of Q1, the output pulse

duration is approximately tW = 1.1RAC. Figure 3 is a plot of the time constant for various values of RA and C. The threshold levels and charge

rates both are directly proportional to the supply voltage, VCC. The timing interval is, therefore, independent of the supply voltage, so long as

the supply voltage is constant during the time interval.

Applying a negative-going trigger pulse simultaneously to RESET and TRIG during the timing interval discharges C and reinitiates the cycle,

commencing on the positive edge of the reset pulse. The output is held low as long as the reset pulse is low. To prevent false triggering, when

RESET is not used, it should be connected to VCC.

Fig. 2 Typical Monostable Waveforms

Fig. 3 Output Pulse Duration vs. Capacitance

RL

RA

THRES

TRIG

DISCH

RESET

CONT

VCC

GND

OUT

Output

4

7

6

2

1

3

8

5

VCC

(5V to 15V)

Input

C