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
To reset the timer the voltage on the reset must be pulled under 0.4 V The reset in put will override other inputs and set the output to low if the reset input is not going to be used it should be wired to Vcc to prevent false signals In its initial state the 555 timers internal flipflop connects the discharge to the ground and sets the output to low Each time the trigger voltage is pulled down under 1/3 Vcc the flip-flop will break the discharge pin’s connection to ground and set the output to high It will hold this state until something forces the threshold pin’s voltage equal to 2/3 Vcc which will reset the flip-flop.
Monostable Mutivibrator and Astable Multivibrator mode
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