ANGEEK - In the time-delay or mono-stable mode of operation, the timed interval is controlled by a single external resistor and capacitor network. Ne555 timer chips are precision timing circuits capable of producing accurate time delays or oscillation.
The threshold and trigger levels normally are two-thirds and one-third, respectively, of VCC. 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. These levels can be altered by use of the control-voltage terminal.