The xx555 timer is a popular and easy to use for general purpose timing applications from 10 μs to hours or from 《 1mHz to 100 kHz. In the time-delay or mono-stable mode of operation， the timed interval is controlled by a single external resistor and capacitor network. In the a-stable mode of operation， the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor. Maximum output sink and discharge sink current is greater for higher VCC and less for lower VCC.

　　For mono-stable 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.