7-Segment Display with Charlieplexing

Overview

The project illustrates a Charlieplexing of a group of 7-segment LED displays which is similar to the Charlieplexing of discrete LEDs but with some changes since all the LED segments have a common pin.

Explanation

In the design, 14 connections are needed to the microcontroller to control 6 displays. But if the charlieplexing technique is used, the connection count will be reduced to 9 regardless of the number of displays. This will result to a bit more complex connections to the displays which is the drawback of using Charlieplexing. By saving 7 output pins on the microcontroller, it will reflect on cheaper price of more lines of code.

The hardware is actually the same as in the standard multiplex sample with only some of the connections being changed. Adding more lines for the segments is the major change involved. On the first display the new line is connected to the A segment, on the second display the B segment and so on. All other segments are connected as usual where all A’s are grouped as well as B’s. The processor has to first output a high value on the transistor line to address display ON.