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NE555

PRECISION TIMERS

Diodes Incorporated. 

NE555/SA555/NA555

PRECISION TIMERS

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.

VCC

(5V to 15V)

RA

Input

C

5

4

CONT

8

VCC

RESET

7

DISCH

6 THRES

2 TRIG

OUT 3

GND

1

RL

Output

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

NE555/SA555/NA555

Document number: DS35112 Rev. 4 - 2

7 of 14

www.diodes.com

February 2012

© Diodes Incorporated

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