What is a Common Cathode Seven Segment Display?

Before we talk about common cathode LED displays, we first have to go over what is a cathode in a typical LED.

Cathode

The cathode of an LED is the negative terminal of the LED. An LED has two terminals. One side is the anode, which gets hooked up to the positive voltage of the circuit. And the other terminal is the cathode, which gets hooked up to the negative voltage or the ground of the circuit.

So now you know that the cathode is the negative terminal of the LED which gets hooked up to ground.

When you have a common cathode seven segment display, which is an array of 7 LEDs, the LED cathodes are all tied together and are common.

This is a typical LED display, with each LED segment labeled with an alphabetical character.

This is how the seven segment display will be wired up in a common cathode setup:

As you can see, all the positive terminals of the seven segment display are all separate. In order to turn any of the segments on, you have to supply 9V to terminal that you want turned on, meaning to each of the terminals. However, on the cathode side, you do not have to hook up each terminal to ground. All of the cathodes of the LEDs are tied together to ground. So once you hook up one to ground, all are hooked up to ground, since they're tied together and are common.

This is a common cathode seven segment LED display.