NE556 IC devices provide two independent timing circuits of the NE555  type in each package. These circuits can be operated in the a stable or the monostable mode with external resistor-capacitor (RC) timing control. The basic timing provided by the RC time constant can be controlled actively by modulating the bias of the control-voltage input. The threshold (THRES) and trigger (TRIG) levels normally are two-thirds and one-third, respectively, of VCC. These levels can be altered by using the control voltage (CONT) terminal. When the trigger input falls below 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. When RESET goes low, the flip-flop is reset and the output goes low. When the output is low, a low-impedance path is provided between the discharge (DISCH) terminal and ground (GND). NE556 IC Datasheet