Texas Instruments NE555 Single Precision Timers are precision timing circuits capable of producing accurate time delays or oscillation.
In the time-delay or monostable mode, the timed interval is controlled by a single external resistor and capacitor network.
In the astable mode, the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor.
The threshold and trigger levels normally are two-thirds and one-third, respectively, of VCC. These levels can be altered by use of the control voltage terminal. When the trigger input falls below the trigger level, the flip-flop is set, and the output goes high. If the trigger input is above the trigger level and the threshold input is above the threshold level, the flip-flop is reset and the output is low. The reset (RESET) input can override all other inputs and can be used to initiate a new timing cycle.