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Topic: 100 LED Matrix Display (Read 10311 times) previous topic - next topic


I cant find any info about crossplexing, or how it is different than mulitplexing, or charlieplexing.

I havnt made up a schematic for my charlieplexing, but I may be able to explain it with text.

Each LED spire is made up of 4 RGB LEDs, they could be common anode or cathode), Each LED has 4 leads, 3 color, 1 common. The leads are played out into a "+" shape, and each LED rotated 90 degrees as it goes from bottom to top, and 4 wired connect the 4 leads on the 4 LEDs. Each of the 4 leads connect to a common, and each of the 3 colors. You connect to one common lead, and any of the other leads to illuminate an individual LED. When you change the common connection, it changes which LED in the spire gets lit, when you change the color connection, it changeds with of the R, G or B LEDs that it lights.


Digikey has a nice Presentation on how cross/charlieplexing works and what the differences are:

The main difference that I see from the datasheet is that the crossplexed drivers can measure the voltage on each LED (as they also have open/short detection)
So a LED failor or a wrong voltage on an LED wont scrapp your matrix.


Thats kinda crazy. You cant really individually control a single LED, you always lite 2 or more LEDs at a time.

I think the open/short detection is a feature found on other LED drivers too, and not specific to crossplexing, but rather specific to using constant current LED drivers.

I guess there is some benefit, in that its lighting multiple LEDS at a time, but it also seems limited, in that you lose some control of what LEDs are lit, in that you cant light one LED without lighting another one too.


The chips are $9 each -
Makes for an expensive board.
Better off with 2 MAX7219s, only $3 each, for 128 LED display.
Parola library good for driving them.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Most if not all the LEDs I've charlieplexed were common cathode, not common anode, and I'm unsure if it works the same with common anode, but it seems like it should.

Of course it does.  In fact, it generally works better because you will use NPN transistors as the emitter-follower buffers to drive the high side to multiplex, so you are driving all colours of each LED at a time.

Now (as a separate point,) someone happened to mention PNP drivers for conventional multiplexing (as common-emitter).  There is an interesting reason for using NPN high-side buffer transistors for Charlieplexing since you are using a negative ground supply system.  By using NPN emitter followers, you connect the collectors to the unregulated (or a least, higher) supply so that the actual current the LED array uses is not drawn from your regulator.  You still of course, lose 0.7V in the Vbe drop in the transistor.

The general rule regarding multiplexing or Charlieplexing, is that you really do not want to use more than an 8-way multiplex to maintain overall brightness.

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