Redirect dynamic indication segments to separate LEDs
Many devices use multi-digit seven-segment LED indicators with dynamic indication. Typically they are connected to a port on a microcontroller or a dedicated driver IC such as the MAX7219.
Sometimes, a separate LED is required in addition to the digital indicator. They can be connected to separate pins of the microcontroller, but you can also save pins. If the numeric indicator has segments that are never used (for example, a dot in the right digit), you can redirect them to an external LED.
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The dynamic indication of the seven-segment indicator is usually to combine all the digital segments with the same name and connect them to the data line of the microcontroller. The common electrode (common anode or common cathode) of each digit is connected to a separate scan line.
In theory, it is possible to connect LEDs in parallel to any segment by connecting the LEDs to the corresponding data and scan lines. But the voltage drop across the external LED is approximately equal to the voltage drop across the indicator segment, so they will both emit light.
To exclude segment emission, it can be shunted with the BE junction of a conventional silicon transistor since it has a lower forward voltage drop than a segment LED. An external LED with a current limiting resistor can be connected to the collector of the transistor.
A separate current limiting resistor is required for the external LED as it allows the LED and indicator brightness to be set to the same brightness, which can have different efficiencies. Figure 1 shows an external LED redirecting the right dot's segment to a common cathode indicator.
Figure 1 shows redirecting the dot segment on the right to an external LED of a common cathode indicator.
This way you can redirect any number of unused segments to external LEDs. From a software perspective, they will be opened in the same way as regular segments.
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