thoughts on my 5x5x5 cube project idea?

[mvmacd]

So I want to eventually build a 5x5x5 led cube. My biggest hurdle is trying to get the multiplexing/power source figured out.

My current idea: link together 2 arduino pro minis (bought on eBay for about $2.20 each, I have 5 or 10 by now) so I can have enough pins to connect each 25 LEDs in each given layer so they can all be sourced on via the io pins, and thus I only have to multiplex with 5 scans.

I haven't figured out a more graceful way to do this, I know it can be done with shift registers (I have 50pcs of the popular 8 bit shift register whose name escapes me atm), but I don't want to wire up all those transistors.

So my only other hurdle, is having enough current sink for each LED. I haven't figured out what color LED I want yet, but I want the current to be roughly 15mA for long life (nothing worse than a burned out led in the middle of a cube that's impossible to get to). That's 375mA of current to sink.

This is my main reason for this post, to ask advice on this point. I have already bought and used the 2N7000 NFET to sink current for each layer on my 444 cube, but with 555 it's no longer within the 200mA absolute max rating of the NFET.

So my idea was to wire the layers normally, 1 common ground for each 25 LEDs in that respective layer, and then connect it to 2 drains of 2 mosfets, and those mosfets have the source go to ground, and have the gate both tied to the same IO pin. Will this work? I know that theoretically it can work because it's like putting 2 AA batteries in parallel to get more current draw. However if it takes too much current from mosfet, I know it could burn it up. I just don't know whether they will balance the load automatically?

Thanks for reading my long post, I can't wait to hear what y'all think!

[CrossRoads]

'328P Promini is good for 400mA (200mA each Vcc pin).
However, there are current limits per port that you must watch out for.

3. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
ATmega48A/PA/88A/PA/168A/PA/328/P:
1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150mA.
2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150mA.
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.
4. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
ATmega48A/PA/88A/PA/168A/PA/328/P:
1] The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA.
2] The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100mA.
3] The sum of all IOL, for ports D0 - D4, RESET should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater than the listed test condition.

I think you will find that connecting all the LEDs together into the cube shape will be a lot more challenging than wiring up 4 shift registers to sink or source current to each column, with a Logic level MOSFET to source (P-channel) or sink (N-channel) for each layer.
N-channel - use AOI514, will handle the current easily.

My preference is P-channel FET to source current to each layer with high current open-drain shift register (TPIC6C595, TPIC6B595) to sink current from each column,
Using 74HC595, is only rated for 70mA on VCC & GND pin, so 8-9mA per IO pin. Poor choice for LEDs, despite many people abusing them for such.

We are making a 9x9x9 cube (7 layers complete so far) using cd74AC164 as current source (24mA outputs) for each column and AOI514 (516? 518? all are similar) as current sink per layer. Cube builder selected common cathode layer due to LEDs having longer anode leads, had to adjust the control board to match.

[CrossRoads]

And the promini regulator is only good for 150mA, so a separate 5V supply for the cube will be needed.