How to Program an LED Digital Display

A seven-segment digital display with a decimal point is a single unit with eight different LEDs — seven linear LED segments forming various alphanumeric characters, and one round LED for the decimal point. Programming a digital display is a straight-forward affair: either send or do not send voltage to the relevant LEDs. There are two different types of seven-segment digital displays, common cathode and common anode, which will need to be wired in different ways.

Find the data sheet for your seven-segment digital display. Determine whether the display is common cathode (CC) or common anode (CA) before locating the pin description. Identify the CC and CA pins; there may be two pins, but you only have to use one or the other.

Connect one of the CC pins to the negative side, or circuit ground, of the five-volt power supply without applying power to the breadboard. If your digital display is common anode, connect one of the CA pins to the power supply’s positive side.

Identify the pins labeled “a” to  “g” and the “dp” pin. Take eight three hundred thirty-three ohm resistors and connect one pin of each resistor to each of the eight pins on the digital display. Plug the other pin of each resistor in an area of the breadboard — as long as it is not connected to anything else — or leave the second pin disconnected.

If the display is common cathode, connect one end of a wire to the positive side of the power supply, leaving the other end disconnected. If the display is common anode, connect one end of the wire to the negative side of the power supply, leaving the other end disconnected.

Power up the breadboard and touch the disconnected end of the power supply wire to one of the disconnected resistor pins. A corresponding segment on the digital display will light up. Repeat the process for each of the segments and the decimal point. If a segment or the decimal point doesn’t light up, check the connections and repeat the test.

Connect eight wires from negative or positive, depending on whether you’re dealing with common cathode or common anode, to the disconnected resistor pins, which should cause the digital display to show the number eight with a decimal point. Try different connections to create different letters and numbers.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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