Need help to gradually ramp up/down intensity of LED at power-on/power-off

[malaybiswas]

I am making a high intensity LED light panel for my aquarium. I got 15 1w Luxeon white LEDs rated at 350mA max continuous current and 3.42V forward voltage.

The LED driver I am planning to use is Xitanium LED-120A-0024V-10F. Here is a link to the datasheet. http://www.luxeonstar.com/xitanium-120v-drivers.pdf. Specification is
Input - 120 VAC 0.3A 31.9W
Output - 7.8-24.6 VDC 1050mA 25.5W

My basic circuit plan is to have 3 leg parallel circuit with each leg having 5 LEDs, so that I have 350mA current on each leg and need 17.1V for each leg of 5 LEDs.

The driver will be connected to power source with a timer to maintain a 10 hr photo period (or anything else that I want).

In this mix I would like to add a circuit that can control the intensity of the lights so that when timer is switched on intensity of lights gradually increase from 0-100% over 1-1.5 hrs time. Then they remain at full intensity till timer goes off. When timer goes off, the intensity decreases from 100-0% over 1-1.5 hrs time.

The ramp up and ramp down periods may not be exactly 1-1.5 hrs. I guess 30 mins can also work (just as long as it can be visibly noticed over a fairly long period of time)

I can do basic circuits, but my knowledge on devising advanced circuits is very limited.

Can anybody suggest me a circuit design to produce this effect for the specifications mentioned? I would prefer something that hopefully does not require too many IC components as I fear that I might riun them by in correctly choosing/connecting stuff. A simpler circuit which might not be as smooth will probably be the best bet for me to try.

Any help is greatly appreciated in advance

 

[berkeman]

You control LED brightness with PWM above the flicker fusion frequency of the human eye and visual system. BTW, in order to series-connect the LEDs, they have to be rated for matching so that they have about the same brightness at the same current. Not all LEDs are rated for that; are the ones you're using rated/sorted for series operation?

Does the driver circuit that you're using have a PWM input control, or some other brightness control hook (I didn't follow the links)?

 

[malaybiswas]

You control LED brightness with PWM above the flicker fusion frequency of the human eye and visual system. BTW, in order to series-connect the LEDs, they have to be rated for matching so that they have about the same brightness at the same current. Not all LEDs are rated for that; are the ones you're using rated/sorted for series operation?

Does the driver circuit that you're using have a PWM input control, or some other brightness control hook (I didn't follow the links)?

Yes these are Luxeon Star white LEDs. They are rated for series operation, and the combination I planned is one of those suggested by Lumileds.

One of the drivers have PWM built into it. Since I don't have enough knowledge yet on PWM, here is the link to the datasheet of the driver http://britelite.com/pdf/a_25w_dimming.pdf .

The driver has 2 control inputs for intensity control. A DC voltage drop across these to terminals can control the intensity. Question is how do I vary the voltage drop over a period of time?