Since the LEDs are now on for only a fraction of the time it is necessary to pulse them with higher current. This reduces the pins required on the controller, reducing cost and circuit board complexity. The pin-count saving increases with larger displays as the pin-count (for a square matrix) is the square root of the number of LEDs. In the example of Figure 1, only seven lines are required to switch twelve LEDs. Each row is switched on in sequence (S1, then S2 and then S3) and the corresponding switches S4, 5, 6 and 7 closed in synch to switch on the desired LEDs. To fool the eye into seeing a continuous display the sequencing is typically done at more than 50 times per second. Current flows through (1), (2) and (3).Ī practical circuit uses tranistorised switches. By closing switches 2 and 6 we can turn on LED 7 alone. In practice some form of current limiting is required. Each LED can be turned on individually by closing the appropriate row and column switches. Multiplexed displays using seven-segment LEDs remain popular due to low cost and high brightness. The advantage is simplification of hardware due to the reduced number of pins required. Multiplexing is a technique used to connect devices – typically LEDs (for displays) or buttons (for keyboards) – in a matrix of addressable rows and columns.
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