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

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

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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)?
 
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 [Broken].

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?
 
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