Inside the 555 timer is a voltage divider the divides +Vcc into thirds A voltage comparator compares the trigger input with 1/3 Vcc and another comparatorcompares the threshold with 2/3 Vcc The point at 2/3 Vcc on the voltage divider is connected to the control voltage This pin can be used to modify the values of 1/3 Vcc and 2/3 Vcc without having to change Vcc However if this input is not going to be used it should be grounded through a bypass capacitor (0.01mF) to protect it from noise The 555 timer also has a flip-flop which is controlled by the two comparators The trigger and reset inputs are level-sensitive active low inputs To activate the trigger the voltage on the trigger pin must be pulled down to under 1/3 Vcc The trigger experiences a delay when changing states so it should be returned to high at least 10 ms before the end of the timing cycle or else the cycle will be immediately re-triggered