led cube 8*8*8 lighting fixture with tlc5940

[shaynet]

i'm working on making a led cube lighting fixture similar to these two:
http://www.instructables.com/id/3D-LED-Cube/
http://www.instructables.com/id/Led-Cube-8x8x8/

and it's supposed to be in my living room and that poses a problem since not only that it has to look nice it must be practical provide enough lite and i figured that 512 leds should be enough, but then i thought a little about it and i and it hit me that it would only give me 1/8 of the light since every led lights up only for 1/8 of the time so i figured that i would have to have a mode in which all of the layers light up at the same time in order to give me the maximum amount of light.
that gave me a problem of current handling
so i figured i would use a similar setup to the one used in the 3d led cube instructable only i would use beefier pnp transistors to the ones used there (i will probably use bc327) so they can provide current to a whole column, i would use n-channel mosfets that would be controlled directly from the arduino to sink every layer.
i decided that i can't use the tlc5940 to sink current since it's limited to 130mA and that wouldn't be enough for 8 leds, and since these leds are paralleled i will control them with voltage and i added a voltage divider and three mje3055 transistors paralleled to handle the current and the power dissipation.
i attached what it my mind is the setup for a column, please tell me what you think about it, tell me if i need to add resistors anywhere especially around the mosfets  and whether it's a good idea to control the leds with the voltage or maybe i'm just stupid and i can get leds that would light differently.
or maybe you have an entirely different idea of how i can achieve my goal, i would be glad to hear about it

[egeltje69]

For the three transistors in parrallel I would use one BC639.
and I'm also working on a 8*8*8 led cube for a friend.
and I use 10 4051 IC's so I don't have any led in parrallel.
and all 512 Led's are controlled seperatly by a PWM signal out of the arduino.
which is going to be replaced by a PIC when done.

[Grumpy_Mike]

When driving LEDs you always need something to limit the current. Either a constant current source or a resistor.
I am not sure I understand the circuit as a whole. Transistors in parallel are a bad idea use a single FET instead.

[shaynet]

the transistors are there to drop the voltage from the supply voltage (5V in my case) to 3.2 or so for the leds so they can work in parallel,
and i paralleled the transistors in order to decrease the current flow and power dissipation (compared to only one transistor).

my problem is that i need to be able to turn on all of the leds at the same time (when using the cube for lighting) as well as being able to turn on only one layer at a time (for the effects). this is why i can't use a resistor for current limiting purposes and the tlc5940 wouldn't handle the current requirements as well as the power dissipation requirements for it to be the current limiting device.

ideas of how to achieve my goals would be extremely appreciated.

[Grumpy_Mike]

the transistors are there to drop the voltage from the supply voltage (5V in my case) to 3.2 or so for the leds so they can work in parallel,

No that is not how it works.

this is why i can't use a resistor for current limiting purposes

Then you can't make your circuit.

ideas of how to achieve my goals would be extremely appreciated.

Change the laws of physics?

[shaynet]

time to change the laws of physics

so i though of a different setup:
instead of the arduino controlling the layers threw n-channel mosfets it will control them threw p-channel mosfets
and columns will be sunk directly by the tlc5940 and current limited by it and that will be used for effects.
for lighting purposes there will also be an n-channel mosfet connected to arduino pin 5 or 6 (for pwm since the other pwm pins and clocks are used for the tlc5940) that will sink threw 1W resistor for every column (for current limiting) the whole cube at once and in this way light all the cube at the same time.
the n-channel will be off and the tlc will be on when the cube will be in effect mode, and then when i need it for lighting the tlc will be off
and the mosfet will be on.
what do you think about this setup?

please help me choose p-channel logic level mosfets (preferably with a threw whole package)

[shaynet]

i really need your help

is there a problem understanding my schematic or my setup that i can clarify?

[egeltje69]

the best way to do what you want is always drive one LED at a time but so fast that it looks like it's completly lit. one current limiting resistor a bunch of analog multiplexers. and no worries on fets, transistors,laws of fysics,etc... just simple and you can do more with the same cube.

[shaynet]

it doesn't matter how fast you cycle between the layers you still cannot have any individual led lit more then 1/8 of the time so you would only get 12.5% duty cycle at best.
that's why i want to be able to light up all of the leds at the same time so it would give me more light so i can use the cube to give light and not only as a nice thing to show to my friends