Aquarium LED light design (some beginner questions)

Thread Starter

Kaisha

Joined Nov 23, 2016
15
Beginner/noob hobbyist here. I've completed a few smaller arduino projects (temperature sensor monitoring of my aquarium, controlling pumps with relays/water level sensors, simple things like that) with success and am now looking to attempt something a little harder. My background is in computer science/programming, so the software side is trivial, but the electronics... well I still have far too much to learn. So feel free to correct any mistakes, or suggest alternatives that would seem otherwise obvious to someone who has a hardware background. I also apologize in advance for mistakes I'm sure to make.

I've looked online for similar projects, but I'd like to do things a bit differently than the usual aquarium led light (which is why I feel a post here is warranted). I'm looking to create a pendant style led light for my aquarium. I plan to control it with an arduino, and I would like anywhere from 8-12 independent pwm channels. I'm going to be running a number of different led colors (full spectrum from almost infrared to almost uv, and everything in between), and I'd like to be able to fine tune the color (and eventually, once the hardware is working, write software to adjust the color continuously over the course of the day). Since these will be on continuously throughout the day, I'd like to avoid resisters/linear regulators for led power management. The idea is once I build one that works, I can then build a few more (depending on how much lighting I need in the aquarium). There were two ideas I had in mind.

The 1st is to use 3w leds, 1 led for each channel. The idea is to drive the leds with a single constant current power source (perhaps something like the Meanwell LDD) and wire them in series. I could then (at least theoretically) turn the individual leds on and off using a transistor as a bypass. Something like this diagram:
Aquarium-Lights.png
I don't know what type of transistors I would need (or if I've even wired them in correctly), but the idea is that each transistor I could connect to an arduino io pin, and use software PWM to turn the transistors on and off. When the transistor is turned on, the line should short circuit turning the led off, and vice versa, or at least that was the idea. The last transistor is to allow turning off all the leds without shorting the power supply. TBH I've never encountered anything like this in my research online, which leads me to believe that its probably a bad idea, but I'm not sure why.

The second idea is to use something like the TLC5940 controlled by an arduino. This seems pretty straightforward, but I would have to use smaller leds. The data sheet shows max current per pin at 120mA and 17V which is just over 2W per channel. If I were to use all 16 channel (doubling up on a few colors) I would get a total of 32W (ish) of light, which is similar to the idea above which has about 36W (ish) of light. Granted I don't intend to be running all of the channels at full power all the time, but I would like the option to do so without blowing anything up. Which brings me to my question, can the TLC5940 handle that sort of power? All the tutorials I've seen show only a few leds running off it (usually or 1 or 2 per channel). I guess I could toss a small heatsink on it, but I'm still not sure if its a good idea to push the chip that hard. Do the data sheets have 'head room' built in? Apart from the ridiculous amount of soldering required, is there any drawback to running say 25-30 or so 5mm leds off a single pin?

I'm partial to the 1st option (or any other idea that you guys may suggest that could handle PWM on anywhere from 8 to 12 individual channels running 3w leds).

Thank-you for your time, and any thoughts or comments appreciated.
 

wayneh

Joined Sep 9, 2010
16,401
I didn't follow all of this but I think the bypass idea is a bad one. For one thing, each transistor would need a different voltage on its base (or gate, for a MOSFET, which is what you want to use) in order to switch. Worse, that voltage would depend on what the other LEDs are doing.

If at all possible, try to make each series string all one color. Then you can control that string with PWM control of a single transistor.
 

Thread Starter

Kaisha

Joined Nov 23, 2016
15
I appreciate the responses. I take it from wayneh's post that the switching voltage for a mosfet is dependent on the source voltage?

The idea came from this chip: http://www.ti.com/lit/ds/symlink/tps92661-q1.pdf at least I imagine something along those lines is happening (Figure 10 on that pdf). In fact that chip seems similar to what I'm looking for, except I really don't understand how I'm supposed to use it. It doesn't seem to actually be a CC power supply, rather just controls and/or interfaces with it somehow?

Any thoughts on the TLX5940?
 
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