The NE555 has a trigger level of approximately one-third of the supply voltage and a threshold level equal approximately to two-thirds of the supply voltage. These levels can be altered by use of the control voltage terminal (CONT). The 555 can be used in countless applications and has three recognised operating modes. In monostable mode the 555 functions as a one-shot or pulse stretcher. Applications include timers, missing pulse detection and bouncefree switches. In astable mode the 555 can operate as an oscillator. Applications include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms and Pulse Width Modulation (PWM). In bistable mode the 555 can operate as a flip-flop or changeover. Applications include bouncefree latched switches.