LED dimming

[aquariumnut]

Alright first post and probably only topic I may author on this forum, but I do need some help, and you guys might be able to help

Essentially I am attempting to make a several hundred LED array to grow aquatic plants in my 29, 10 and 5g tanks. The 29 will most likely have well over 300+ LEDs, the 10 with about 125 or so depending on the success of the 29, and lastly the 5g hex with about 50-75 depending.

I've read reports and LED growth far outstrips PC (power compact lighting) watt for watt when using high power LEDs. However I dislike high power for the simple matter of heat. I've read where someone stuck 900 LEDs over a 25g reef tank, over half of consisted of blue and UV LEDs that a freshwater will not need. LED lighting is here, and just as much as T5VHO lights. Might as well get the best.

Oh, did I mention I will not have to replace this system until 2018?

The LEDs will be mounted on a 12"x30" piece of acrylic (leftover from something else). I will be using these btw, http://www.superbrightleds.com/specs/W18030.htm

The plan is to make 134 strands with 3 LEDs in series each @30mA each strand and 134 resistors to prevent frying LEDs. Powering it will be a used computer PSU, which can supply enough power on its 12v rails for all three tanks. Now this is great for growing plants, But I'd like to be able to dim all these LEDs at night. Heck, programing a chip to essentially create a sunrise/sunset effect at specific times would super neat. If that cannot be done under $100, fine. But in any case I would like to able to view my aquatic garden with minimal light during the night. Reds LEDs will be included to help give a more balanced spectrum to the plants (they need mainly reds and blues, and the above does not provide enough red). Now these will have individual strands themselves out of the 134, this is great because it would give a better sunrise/sunset effect.

On the subject of dimming, if a controller can't be built, would it be possible to dim all 134 strands with a Darlington transistor? I have one that can handle 5A of current, so it could handle it, question is would it work?

Ideas please, this is in the planning stages and I'd rather get this hashed out now then have to redesign down the road.

[Alan]

Great sounding project. We also sell LEDs but I am not sure that the wavelength would be ideal for plant growth.
http://alan-parekh.vstore.ca/index.php/cPath/4

If you just want a simple dimming circuit I would just use a 555 timer and your darlington to switch the power.

[aquariumnut]

Forgive me for never delving into 555 timers, I'm tired at the moment so understanding them is difficult at the moment. However I doubt this would be able to create sunrises/sunsets (micro-controller perhaps?) but would be able to dim it.

I was thinking originally that the transistor would handle the current while a pot or rheostat controlled the intensity. I know this works with standard DC lightbulbs, but I question how well it would work in an LED setting.

[Alan]

I think I get what you mean. I was thinking that you would have a pot that could adjust the intensity of the LEDs using the 555 timer. If you want it to work automatically then I would suggest using a microcontroller to make it work.

[aquariumnut]

to which I lack programming skills and tools, Though I suppose someone could program one for me.

Thought though, Could a photoresistor do the job of dimming along with the weather I have. Although this does present a problem as far as location (20'+ in total wire length from positive to negative, thus increasing resistance) I'm thinking it may work.

So would anyone here be willing to program the chip for me? If not, would a photoresistor be useable?

[Alan]

I think to get the results that you are wanting the photocell would have to be read by a microcontroller and taught what to do based on the input levels.

It is a simple project but I think you would probably need to learn a bit of coding and build it yourself since it would be quite time consuming for someone to build.

Have a look here:
http://www.hollysolar.com/html/control.html
I wonder if any of these would work for you?