LED Displays

LED displays are display devices that use light emitting diodes, or LEDs, to light the segments. These usually come in red, but other colors, such as green or yellow, are also available.

Configuration for an LED display is usually in a seven-segment "rectangular figure eight" pattern, but other patterns are available, such as the "Union Jack" fourteen- and sixteen-segment arrangements. Five-by seven and eight-by-eight matrices of LED dots are also common. Also, there are two ways these are internally connected: common anode, with all positive ends of the LEDs hooked to one power pin and negatives serving as the segments connections, and common cathode, with the polarity reverse from common anode.

First off, you will need the terminal assignment, as shown in the image below.

You will need to know which pin is which, so each pin is numbered... the table below will show which number goes to which pin.

Now, to light a number in the display, you need to light a set of segments... for 4, you need to light B, C, F, and G, for example. The image below shows sets of digits, 0 through 9, that show two ways the 6 and 9 can be displayed.

To properly drive these, though, you will need a special IC to illuminate the appropriate segments for each number. Here, we will use the 74LS48 BCD to seven segment decoder with a common cathode display. This IC takes a 4-bit binary input and converts it to a seven-segment output.

Since this IC outputs a full 5 volts, we will need to put resistors in series with the outputs. 270 ohms will work here. If we omitted the resistors, our LED display would fail under too much voltage, and may even explode!

Once our LEDs are connected to the IC through resistors, we can start connecting the inputs. Connect the four BCD inputs to any device that you wish to acquire a digital 0 to 5 volt signal from, and tie the Lamp Test and Blanking pins to the +5 volt rail. You should see the display light. If so, congratulations, you have successfully interfaced an LED display!