N555 datasheet(9/27 Pages) TI

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APPLICATION INFORMATION

Monostable Operation

VCC

(5 V to 15 V)

Output

GND

OUT

VCC

CONT

RESET

DISCH

THRES

TRIG

Input

Pin numbers shown are for the D, JG, P, PS, and PW packages.

NA555, NE555, SA555, SE555

PRECISION TIMERS

SLFS022F – SEPTEMBER 1973 – REVISED JUNE 2006

For monostable operation, any of these timers can be connected as shown in Figure 9. If the output is low,

application of a negative-going pulse to the trigger (TRIG) sets the flip-flop (Q goes low), drives the output high,

and turns off Q1. Capacitor C then is 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 flip-flop (Q goes high), drives the output low, and discharges C through Q1.

Figure 9. Circuit for Monostable Operation

Monostable operation is initiated when TRIG voltage falls below the trigger threshold. Once initiated, the

sequence ends only if TRIG is high at the end of the timing interval. Because of the threshold level and

saturation voltage of Q1, the output pulse duration is approximately tw = 1.1RAC. Figure 11 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.

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