The 555 family of timer chips can be used in either monostable Multivibrator mode or astable Multivibrator mode In the monostable Multivibrator or “ one shot” mode each time the 555 timer is triggered the output will go high for a specified amount of time then return to low and await another trigger signal In the astable Multivibrator mode the timer triggers itself periodically and becomes an oscillator sending out a train of pulses