In a LED 7 segment display, as the name indicates the 7 segments plus the dot segment are based on LEDs. When power is given to a particular segment, it glows and the desired digit can be displayed by powering the suitable combination of LEDs. LED seven segment displays are of two types, common cathode and common anode. In a common cathode display, the cathode of all LED segments are tied together as one common cathode pin and the anode terminals are left alone as input pins. In this scheme, the common cathode is always connected to ground and the control signals (active high) are applied to the inputs (anode terminals). In common anode type display, the anodes of LED segments are tied together as one common anode and the cathode terminals are left alone as input. In this configuration, the common anode is always connected to a suitable positive voltage and the control signals (active low) are applied to the inputs (cathode terminals). Pin out and image of a seven segment display is shown in the figure below.