Charlieplex LED Failure Modes

December 23, 2018 at 2:28 pm (computers) (, , , )

I’ve been playing with Charlieplex LED arrays for a while now, and have been linking up my range of 8×7 LED Array and MCP23017 modules to create a larger display.  However, I’ve had that fault you sometimes get with Charlieplex LEDS where instead of a single LED lighting up, you get a good chunk of a row and a column lighting up at the same time.  I wanted to understand why.

From Wikipedia:

If the failed LED becomes an open circuit, the voltage between the LED’s 2 electrodes may build up until it finds a path through two other LEDs. There are as many such paths as there are pins used to control the array minus 2; if the LED with anode at node m and cathode at node n fails in this way, it may be that every single pair of LEDs in which one’s anode is node m, cathode is p for any value of p (with the exceptions that p cannot be m or n, so there are as many possible choices for p as the number of pins controlling the array minus 2), along with the LED whose anode is p and cathode is n, will all light up.

To understand what this means, I took the excellent Sparkfun tutorial on their Charlieplex array, and annotated the diagram to show what happens if the LED for P1->P2 fails with an open circuit, following the Wikipedia description:

Charlie-failure

So instead of the flow from P1 to P2 directly, illuminating D1, the current finds a path via the common connection to P3 between P1 and P2, thus illuminating both D2 and D6.

When this is expanded out to a whole array, you can see how the current finds a path through all pairs of LEDs sharing the anode and cathode, via another pin, with the failed LED.

If you are scanning the LED with a persistence of vision type arrangement, this kind of thing might be hard to spot other than some general weirdness in the display.  The key to finding out which LED is at fault is to slow down the scanning and illuminate each LED in turn.  Of course, whether it is possible to fix or not will depend on your LEDs and soldering ability…

Wikipedia describes other modes: closed circuit – in which case, presumably you’ll start to get pairs of LEDS lighting up (as if both P1 and P2 are joined); and “leaky” where there is current flow in both directions, in which case you presumably see a combination of both of these failure modes.

Charlieplex displays are great … but when they go wrong, they go wrong fairly completely.

Kevin

 

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