7 Segment Display Solver / Display Decoder

Today we will discuss another important topic of our series, 7 Segment Display Solver / Display Decoder.

One digital decoder IC is a device that converts the digital format to another. To do this, one of the most commonly used devices is called a 7-segment display decoder with binary decimal (BCD). 7-segment LED (light-emitting diode) or LCD (Liquid crystal display) type displays provide a very convenient way to display information or digital data in the form of numbers, letters or even alphanumeric characters.

Typically, a 7-segment displayconsists of seven separate color LEDs (called segments) in a single display package. The correct combination of LED segments must be illuminated on the screen to produce the required numbers and letters from 0 to 9 and A to F respectively.

A standard 7-segment LED display usually has one for each LED segment and eight (8) input connections that act as a common terminal or connection for all internal display segments. Some single screens also have an additional input pin to display a decimal point in the lower-right or left corners.

Electronics have two important type 7 segment LED digital displays.

  • Common cathode display (CCD) – On the common cathode screen, all cathode connections of LEDs are combined into logic "0" or soil. Individual segments are illuminated by the application of the signal "high", logic "1" to individual anode terminals.
  • Common anod display (CAD) – On the common anode display, all anode Connections of LEDs are combined into the logic "1". Then individual segments are illuminated by connecting the individual cathode terminals to the "low" logic 0 signal.

Common Cathode and Common Anod Format

7 Segment Display Solver
Common Cathode and Common Anod Format

7 Segment Display Format

7 Segment Display Solver
7 Segment Display Format

Therefore, for example, to display the number "3", segments A, B, C, D, and G need to be illuminated. If we want to display a different number or letter, a different set of segments needs to be illuminated.

Accuracy Table for 7 Segment Display

7 Segment Display Solver
Accuracy Table for 7 Segment Display
7 Segment Display Solver
7-segment display elements for all numbers.

It is conveniently seen that we will need seven separate segment links and an additional connection for the "common" connection of the LED to display any single digit number from 0 to 9 or letters A through F. In addition, since the segments are basically a standard light-emitting diode, the drive circuit must produce up to 20ma of current to illuminate each segment and display the number "8". This causes a total current of approximately 160mA (8 x 20mA).

Obviously, the use of so much connection and power consumption is impractical for some electronic or microprocessor-based circuits. Therefore, decoder and Driver ICs are used instead of screen decoders such as BCD to 7-Segment Display to drive only one screen and reduce the number of signal lines required.

Binary Encoded Dex Numbers

7 Segment Display Solver
Binary Encoded Dex Numbers

From BCD to 7-Segment Display Decoders

The 7-segment display decoder(BCD), such as the TTL 74LS47 or 74LS48, has 4 BCD inputs and 7 output lines, one for each LED segment. Allows the use of a smaller 4-bit binary number (half-byte) to display all decimal numbers from 0 to 9. By combining two screens, a series of numbers from 00 to 99 can be displayed with a single byte of eight data bits.

From BCD to 7-segment decoder

7 Segment Display Solver
From BCD to 7-segment decoder

The use of packaged BCD allows two BCD digits to be stored in a single byte (8-bit) data. Allows a single data byte to hold a BCD number in the range 00 to 99.

The following is an example of a 4-bit BCD entry ( 0100 ) representing the number "4".

Screen Decoder Example

7 Segment Display Solver
Display Decoder Example

In practice, approximately 150Ω to 220Ω current limiting resistors will be serially connected between the decoder/drive chip and each LED display segment to limit maximum current flow. Different display decoders and drivers are available for different types of displays, including LED or LCD. For example, 74LS48 for common cathode LED types, 74LS47 for common anod LED types, or CMOS CD4543 for liquid crystal display (LCD) types.