75 Gallon LED Hood

[shawn]

I'm curious what frequency of PWM you are using? Has anyone done a study on corals and PWM to understand the effects? Is there an optimal frequency (my guess would be that higher is better)? I have no idea if photosynthesis and human vision have any similarity with respect to PWM response - or for that matter, other light sensing sea life.

 

[phalanx]

There are 2 predominant frequences in this particular controller. The first one is the switching frequency of the FET used to maintain constant current control. It is based off of a fixed off time generator set to about 2.3uS. On time can vary depending on the applied voltage and inductor impedance so the overall switching frequency I estimate to average somewhere inbetween 100-200Khz. This is in a constant conduction mode so the LED never shuts off during this switching.

The other frequency I use is for PWM dimming at 150Hz. Other than looking at the results of other peoples' tanks with LED setups (which are fine by the way), I don't have any empirical data to show the effects of PWM on photosynthesis. The only reasoning I can muster is that photosynthesis is a biological process which like other biological processes (think human vision) has a very slow response time relative to the switching frequency of the electronics and filters out the rapid on/off switching.

The units for PAR is umol photons/m^2/second which means the quantity of photons hitting a specific area over a specific amount of time. An LED running at half intensity all the time will have the same PAR as an LED running at full intensity half of the time.

 

[sciknen]

this is awesome.. Didn't even see this till now.

 

[shawn]

I poked around a bit and found at least one study done with terrestrial plants (I didn't find anything related to marine environments) that found that higher frequencies (~750KHz) were optimal. That said, it wasn't that big of a difference, at least as far as I could tell. I do wonder if some animals with advanced eyes (such as mantis shrimp) have a different tolerance for "flicker" than we do.

By the way, thanks very much for the detailed information on the project - it is very interesting!

 

[MrReefer]

Bump! Any updates? Looking forward to see final product

 

[phalanx]

I just received 100 lens assemblies just in time for me to go on another business trip so it will be at least a week before any real progress is made. I've been working on the timer/dimmer in the mean time and progress is good. It will let you pick times and intensity for each color. Once I groom the code some more I'll have smooth gradual intensity transitions to better mimic daylight.

I've also been experimenting with a moonlight mode but I'm not sure how well it will work due to the minimum brightness levels of 50 royal blue LEDs.

I will keep this thread updated but it will be around my work schedule.

 

[phalanx]

Lenses are mounted! I was able to get rid of the glitch switching between high and low current mode. I have a working moonlight mode and will shortly have gradual transitions for sunrise and sunset times. As usual, my cell phone doesn't know how to deal with royal blue light so the exposure is messed up.

 

[NeverlosT]

looks good man. I think that your intuition regarding biology is correct. At the rate that you are switching these electronics on and off, it is going to be completely transparent to any biological critter, and should have no effect of photosynthesis, energy is energy.
That is going to be a SUPER powerful system for a 75g normal depth tank, in fact, the end user may consider using it without optics. They will definitely want to start in the low drive dimming range at first to let the critters adapt. We all must remember that light energy usable by our corals does not = light energy visible to us, so even something that doesnt look as bright as a halide bulb can badly burn ones corals.

I think that your build is awesome, reminds me of our projects in engineering school, I want to build a dimmer circuit and program it! What are you using to program the dimming and will the boards be microcontroller controlled or will they plug into a controller for their dimming input?

Very nice work!

 

[phalanx]

Very nice work!

Thanks! It's taken longer than I would have liked it to but there isn't much I can do about the schedule of my day job. I'm thankful that the person I'm building it for is understanding as well.

You must be the NeverlosT on NESR. I'm over there as ZX-12R and I'm building this fixture for number1gsxxr who is also on NESR as Punjistick. It's a small world!

The current controller ICs I used have inputs for PWM and analog dimming. I interface with them through the optocouplers on the driver boards. To generate the PWM signals, I'm using a Microchip PIC24FJ64GB002 microcontroller. It's a 16-bit, 16 MIPS controller that has multiple PWM generation units built into it. It also has an RTC module but I'm using an external one because interfacing a battery back-up is easier with a discrete part. My controller has a serial port that you can connect to with a laptop or PC so you can set the time, program schedules, and control the properties of the light. I use MPLAB as my development environment and I use the Microchip C30 compiler (which I have a commercial license for) to write the majority of my code. I program the part using a RealICE.

I'm sure the light will be overkill at full power but part of the design philosophy was to run the LEDs at less than full power to prolong their life and eliminate the need for cooling fans on the heat sink. I've let the array run at full power overnight with the heatsinks upside down and it does get fairly warm but not to a point where I can't touch it. I've also been conservative in my design because it's going to be used by someone other than myself so I need it to be as reliable as possible.