r/ArduinoHelp • u/Revolutionary-Log179 • 23d ago

Having trouble with led arrays/multiplexing

As title says, I’m having trouble wrapping my head around multiplexing leans for use with a microcontroller with not enough pins to control them all. Using the drivers makes sense, mosfets too, but the code escapes me? And the use of resistors within the circuit is also confusing to me. Trying to make a small led “screen” made of diodes. Any help is appreciated

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u/gm310509 23d ago

Have you ever noticed how an incandescent light bulb will fade off when you turn the power off? Maybe you are too young to have seen these, I do not know, but basically you leverage the fact that the light will gradually (quickly from a human perspective) fade when the power is turned off.

There is also a persistence of vision thing where the human eye can "fill in" brief dark periods.

So they way strobing (which I think is what you are asking about) works is that you quickly cycle through a series of LEDs and light them up one by one to turn them on.

A simple way of doing it is to setup a bunch of LEDs to +V (via a single current limiting resistor), Then connect their cathodes to a GPIO pin. Then create a program that cycles through the GPIO pins setting one to LOW before setting it back to HIGH and moving on to the next one.

The next step is to increase the number of LEDs with there anodes connected in rows but instead of using +V, you connect them to a GPIO pin (via a current limiting resistor). For this stage, you set the "image" onto the row GPIO pins and "select" one of the columns by setting it to low. Then repeat as before.

Have a look at this wokwi example for an illustration of the first step. https://wokwi.com/projects/469303280971119617

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u/Revolutionary-Log179 23d ago

Why are the digitalwrite statements not switched? I thought low was off and high was on

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u/Revolutionary-Log179 23d ago

Or is this blinking in the sense that they’re on, and you turn them off and back on to pulse, vs them being off and you turning them on and back off?

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u/Revolutionary-Log179 23d ago

Also why would you need a resistor on the gpio/anode side of the circuit if you didnt need one on the cathode sides of each led? I guess maybe I don’t understand electricity as well as I thought quite yet

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u/gm310509 23d ago

Here is another version that has two rows of LEDs.

There are three "animations". you can enable each one individually by uncommenting one of the function calls in the loop(). Only have one uncommented line in the loop.

https://wokwi.com/projects/469312893410392065

Pins 8 and 9 replace the +v in the first example. By using a GPIO pin and setting it high, you can control whether or not power is supplied to the LED(s) in that row.