Introduction to 7-Segment LED Display
A 7-segment LED display is used to show numeric values. To show 3 decimal digits, we need 3 units of this display. Each unit has 9 connectors, 1 for common anode or cathode, 7 for the segments, and 1 for the dot.
For common anode type, to light all segments we have to tie the common anode pin to the positive terminal of a voltage supply, then connect each segment’s cathodes to ground through current limiter resistors (one resistor each for each cathode). For common cathode type, similar method applies: tie the common cathode pin to the ground of a voltage supply, then connect each segment’s anode to the positive terminal of voltage supply through current limiter resistors.
Multiplexed 7-Segment LED Display
If we need 3 decimal digits display, then we need 3 unit of 7-segment display. When driving this display using a microcontroller, ideally we need 21 digital outputs from microcontroller pins to drive all 21 segments (without the dots) independently. Unfortunately most microcontroller has limited number of IO pins, so we have to drive the display with multiplexing technique to use fewer pins. Multiplexing is done by turning two or more displays sequentially, for example start the process by turn on the display_1 for 10 milliseconds, turn display_1 off and turn on the display_2 for the next 10 milliseconds, turn display_2 off and turn on the display_3 for the next 10 milliseconds, turn display_3 off and restart the process again and again. Using this technique, now the 7-inputs of the 7 segments from each 7-segment unit can be paralleled. For 3 decimal digits display, only 3 digital output pins is needed to independently activate each display by driving its common anode/cathode pin. The rest, the segments (7 lines) driving is applied together in parallel for all 3 units. See the software implementation here: http://www.deeptronic.com/arduino-seven-segment-display-multiplexing-state-machine-implementation/
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