deʃhipuOnce I got my matrix-driving code working on the PewPew 8.0, it was just a question of hacking a tri-state gpio pin switching function and some trivial modifications to get it to work with a charlieplexed display. At least it seems so, because it looks like it's working:
There is a slight problem with brightness — they are way too bright now to distinguish individual shades. Of course my phone camera got overloaded and did something funny with the colors, to the naked eye they are all blindingly red. I will need to tune the PWM somewhat.
I'm powering it with a bench power supply for testing, because the poor little coin cell only lasts a few minutes with the display on at full power — even though the bench supply claims it's only drawing 20mA total.
We will see what I can do with it, but it definitely looks promising.
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Bench supply can only see *average* current. In reality your multiplexed display currents would have much higher peak current pulses. High current x high internal resistance of coin cell = high voltage droop.
Use large values low ESR bulk capacitor would help to supplement the high current pulses. You can figure out the value from the Q = CV equation.
Switching to the rechargeable version of the coin cell would help as they have much lower internal resistance.
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It's already a rechargeable version. The regular version wouldn't stand a chance.
I have a 2µF capacitor in there already.
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2uF is hardly a large value. Probably needing hundreds uF.
If you don't have a scope, the LED peak current can be approximate from battery voltage, battery internal resistance, driver internal resistance and LED forward voltage.
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that's actually a good start for a charlie-plexed matrix - too bright LEDs :)
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I guess the opposite problem would be much harder to solve
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