Logic Gates Puzzle 10

The shown logic circuit performs a handy function; can you discover what it is?

The circuit was not optimised for functionality, but to create a nice artwork and interesting puzzle.

If you need to know the meaning of the used logic gate symbols, have a look at this Wikipedia page. Also, there are great videos from Ben Eater on how to build real digital logic circuits.

If you get stuck, check the hints below.

Hints

Hint 1

The overall shape of the circuit is a hint.

Hint 2

Have a look at the outputs, why they have letters and not numbers?

Hint 3

A real circuit like this would have four inputs, not only three. It makes it so much more useful.

Hint 4

The outputs are connected to seven LEDs which are arranged in a special way.

Solution

This circuit is a 7-segment display driver with only 3-bit input. It displays the digits 0 to 7 on an attached 7-segment display.

You find more details about 7-segment displays on this Wikipedia page. Here you also find the common assignments of the letters A to G to each segment.

Real-World Design with Matrix

To build a complete 7-segment driver, you start with a binary decoder. The decoder will convert the binary encoded numbers from 0000 to 1111 into 16 distinctive signals.

Then you create a matrix from these 16 signals and the seven outputs. If a segment should light up for one of the 16 distinctive signals, you connect it using a diode.

For a wonderful example, see Fran’s great video about diode steering.

Additional Challenges

The circuit displays the digit 7 in an alternate form, adding the segment F. Do you find out, why this decision was made?
Design a circuit that displays a signed 4-bit number, including the existing digital circuit. The minus sign will be an additional output I_minus.