Terminals of popular 555 timer IC

Terminals of the 555 Timer IC are numbered starting from the left side of the notch as shown in the image. There are a total of eight pins and we will see the naming and function of each pin.

Pin 1: Ground terminal: Voltages of all other pins are measured with reference to the ground terminal.

Pin 2: Trigger terminal: When the voltage at the trigger terminal is greater than one third of the supply voltage Vcc, the output will be LOW. If the voltage is less than one third of Vcc, the output will be HIGH.

Pin 3: Output terminal: As mentioned above, output can be LOW or HIGH, depending on the voltage at the trigger terminal.

Pin 4: Reset terminal: The timer can be reset by applying a negative voltage at this pin. When not in use, it is connected to the Vcc or supply voltage Pin 8, to avoid false triggering.

Pin 5: Control voltage terminal: Trigger voltage can be changed by applying an external voltage to this pin. Without an external voltage, the threshold voltage at this pin is 2/3 of Vcc because of the connection between the second and third voltage divider resistors with respect to the ground terminal. Usually a 0.01 μF capacitor is connected from this pin to the ground to bypass any supply noise.

Pin 6: Threshold terminal: Threshold terminal gives the non-inverting input to the upper comparator. When the voltage at this pin is 2/3 of Vcc or more, the output of the upper comparator becomes HIGH. This leads to switching output of the timer to LOW. Threshold terminal is typically connected to the ground through a 0.1 μF capacitor.

Pin 7: Discharge terminal: Discharge terminal allows discharge of the 0.1 μF capacitor through an external resistor when the output is LOW. It is connected to the collector of the discharge transistor.

Pin 8: Supply terminal: Vcc or supply voltage between +3 to +18 V can be given to this terminal of IC 555.