LM555 Timer

Precision monolithic timing circuits capable of producing accurate time delays or oscillation. In the time-delay or monostable mode of operation, the timed interval is controlled by a single external resistor and capacitor network. In the astable mode of operation, the frequency and duty cycle may be independently controlled with two external resistors and a single external capacitor.
The threshold and trigger levels are normally two-thirds and one-third, respectively, of VCC. These levels can be altered by use of the control voltage terminal.