The 555 timer chip works by distributing electric current between eight different pins, typically arranged in a rectangle with four on each side. Electricians connect the pins to a circuit to produce timed events in series.
The first of the eight pins on a typical 555 timer chip is the ground pin. The second pin is an active low trigger, which starts the 555 timer when the voltage on the pin drops below one-third of the supply voltage. The third pin is the output pin. It either produces a low or a high output, based on the activity of the other pins. For instance, when pin 2 activates the 555, the output on pin three becomes high voltage.
The 555 timer chip's fourth pin restarts the chip's timing operation using an active low input; it must be connected to the supply voltage for the chip to operate. The fifth pin is the control pin. In most cases, it is wired to ground through a small capacitor, leveling out fluctuations that would otherwise affect the chip's behavior. The sixth pin monitors the voltage discharged by the seventh pin, ending the timing cycle when it reaches two-thirds of the supply voltage.
The seventh pin discharges an external capacitor; it is typically wired to the supply voltage via resistor and to ground via capacitor. The last pin is wired to a positive supply voltage of between 4.5 and 15 volts.