Digital pinball - LED fade circuit

Hope someone can think outside the box to help with this board I would like to make.

I have a digital pinball machine that has high powered Cree LED's on top that are activated by the emulator on the computer. It gives the output via a board called a LEDwiz, which just drives 32 outputs via darlington pair transistor chips ULN2803 controlled by USB. These outputs are then used as inputs on a 'booster' board with N-Channel Mosfets doing all the high power work.

There is a few of us with this setup and would like to be able to make the LED not so harsh in the way it flashes by fading in and out, but also have it variable for tweaking.

I started with a variable resistor and capacitor onto the gate of the mosfet, but I think the pull down resistor that is connected to the gate also is draining the RC circuit too quickly.

Is my theory totally off? Are my electronics skills that rusty or am I missing something?
Any other suggestions of a way to acheive this?

Apprecitate any help

Your problem might be that the difference in gate voltage between OFF and ON states is probably not that great. If you look at the transfer curve (Id vs Vgs) you'll see that Id changes dramatically with small changes of Vgs. The 'better' way to control the brightness is by pulse-width-modulation, but I'm guessing that's too involved for your application. You might try using BJTs rather than MOSFETs, they might work better with the RC approach.

Thanks very much for responding Barry. I was afraid that might be the case.

The 'booster' board we are using is designed around the mosfets so I dont know if I would have any luck changing them out.
PWM is possibly too involved but I will start down that path so see what I can come up with.
Time to break out the breadboard and thinking cap (more commonly known as beer)