LEDs provide a convenient way to electronically display information. Although the seven-segment LED display, arranged in the form of the digit 8, is common, it does not allow the display of some alphanumeric characters. A 5×7 LED matrix allows the display of all ASCII characters, as well as graphics shapes. The circuit in this Design Idea shows an unconventional way to use a 5×7 LED matrix.
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| Figure 1. | Use one 5×7 LED matrix horizontally to make a two-digit display. |
You can use a design containing a set of 5×7 LED units without changing anything in the circuitry, except for the arrangement of the LED units. Using one 5×7 LED matrix, or N units, horizontally instead of vertically allows the display of two characters, or 2×N characters. The minimum pattern for lowercase and uppercase letters requires only a 3×5 LED configuration, except for the letters M and m, which require at least a 5×5 LED configuration and need a dedicated subroutine. The circuit in Figure 1 uses an 8-bit, 18-pin PIC microcontroller and a decade counter to drive one or two 5×7 LED units to provide a display module of two or four digits. The circuit uses a small pushbutton switch to increment the counter. By default, the circuit works in high-brightness mode. If you press the pushbutton during power-on, the circuit works in low-power mode.
You can download two assembly source codes that take up fewer than 256 words. Each uses a top-down scan-line multiplexing technique in a high-brightness mode, in which you can turn all LEDs in a scan line on or off, and a lowpower mode, in which you can turn only one LED at a time in a scan line on or off.
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