NE555/SA555/NA555PRECISION TIMERSTypical Applications CharacteristicsMonostable 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.