Seven Segment Display Example 1

Here is a simple decoder implementation of a seven-segment display to show the letters "H", "S", and "L". There might be an occasion when you need to implement a display to indicate temperature, and H could mean high, L for low, and S for safe. Suppose a device produces binary outputs X and Y depending upon temperature and you need to display this information in a meaningful way using a seven-segment display. Then you will require a decoder that will take X and Y as inputs and provide the necessary outputs to drive the segments of the display. To design the decoder logic, the first step would be to make a truth table showing which segments would have to light up, which is straightforward.

In the table above, X and Y are the inputs to the decoder circuitry, and the rest of the columns are the required outputs to light respective LED segments. The column "a" sequence is 0110, and straightaway you can see that this will require an XOR gate to produce that sequence given X and Y as inputs. The XOR gate is that strange one where both inputs X and Y are "1", to produce an output of "0".

Column "b" is a simple AND function of the X and Y inputs, and column "c" an OR function. For column D you might have to think a little but it is similar to column "b" except inverted, hence this is a NAND function.

Comparing column "a" and column "e", we see that they are complementary -- their outputs are opposite -- therefore, this is an XOR followed by a NOT, making it an XNOR function.

Segment "f" remains ON all the time, hence we require a logic 1 state there. We assume this circuit is TTL logic, and therefore we tie it to +5 V.

Comparing columns "c", and "g", we see that they are identical, requiring an OR function.

Using a common cathode display is the easiest because logic 1 state provides 5 V to light the segment.

Logic Circuit Diagram

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