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Light Emitting Diodes (LED), as the name implies,are diodes that emit light when forward-biased. LEDs are used to display information. Commonly-used LED displays are offered as seven-segment displays and dot-matrix displays. The seven-segment display is arranged in the form of the digit 8 and can display numbers and hexadecimal characters (0F).A seven-segment display is a group of eight LEDs arranged in segments. These segments are generally used to display the numbers 0 through 9 and the letters A through F.


For a certain character, a combination of LED ON and LED OFF is generated to display the character for a short period of time. The pattern is loaded alternately to display other characters. For example, to display the number 2, LEDs a, b, d, e, and g are illuminated.


To control four 7-segment displays, multiplexing canreduce the number of GPIO pins required.In this setup, the four multiplexed seven-segment displaysare turned on one at a time to output the appropriatedisplay. Because of the visual phenomenonknown as persistence of vision, rapid switching of theseven-segment display can appear as if all four displaysare turned on.




The table illustrates the procedure for displaying afour-digit value in the multiplexed seven-segmentdisplay. In this example, to be able to display thevalue 1258, number 8 is displayed first by inputtingthe pattern of number 8. Next, the NPN transistorconnected to the corresponding seven-segment displayis switched on in a short period of time while theother transistors remain turned off. Next, the displayis turned off, and the following pattern is input to turnon the second transistor. This pattern repeats until thefour digits alternately display. Rapid switching producesthe illusion that all four 7-segment displays areturned on.