This Design Idea shows how you can redirect any number of unused segments to external LEDs.
Many devices use multi-digit seven-segment LED indicators with dynamic indication. Usually they are connected to the ports of the microcontroller or to specialized driver ICs, such as the MAX7219.
Sometimes, in addition to the digital indicator, separate LEDs are required. They can be connected to separate pins of the microcontroller, but you can also save pins. If the digital indicator has segments that are never used (for example, a dot in the rightmost digit), they can be redirected to external LEDs.
With a dynamic indication of a seven-segment indicator, usually the same-named segments of all digits are combined and connected to the data line of the microcontroller. The common electrode (common anode or common cathode) of each digit is connected to a separate scanning line.
In theory, it is possible to connect an LED in parallel to any segment by connecting it to the corresponding data line and scan line. But the voltage drop across the external LED is approximately equal to the drop across the indicator segment, so they both will glow.
To exclude the segment glowing, it can be shunted with the B-E junction of a conventional silicon transistor, since it has a guaranteed lower forward voltage drop than a segment LED. An external LED with a limiting resistor can be connected to the collector of the transistor.
A separate limiting resistor for the external LED is desirable as it allows the brightness of the LED and indicator to be set to the same, which can have different efficiencies. Figure 1 shows the redirection of the segment of the rightmost point to the external LED for a common cathode indicator.
Figure 1 The redirection of the segment of the rightmost point to the external LED for a common cathode indicator is shown.
In this way, you can redirect any number of unused segments to external LEDs. From a software point of view, they will turn on in the same way as regular segments.
This article was originally published on EDN.