555 timer

The 555 timer is commonly used as an astable multivibrator, a monostable multivibrator or a voltage-controlled oscillator. It consists of two comparators, a resistive voltage divider, a flip-flop and a discharge transistor. It is a two-state device whose output voltage level can be either high or low. The state of the output can be controlled by proper input signals and externally connected time-delay elements.