I've been planning to build a drop in UAS for the back of a BioCube29. This final design is still in progress; testing some ideas has started on a separate feeder tank. The goal is to have a pair of screens crammed in the rear compartment to allow alternating cleaning cycles. To maximize the width of the water channels, the lights need to be as thin as possible as the width of the rear chamber is about 3 inches wide. Current concept is to use 0805 sized SMD LEDs on a custom PCB.
Testing some ideas for the final design has started in a 20 gallon feeder tank with a DIY hang on glass UAS. Tank is about half filled with brackish water (16 ppt), 72°F. No tank lighting. About 1 inch of aggregate. About 1g of pellets per day. No algae in the tank. Nitrates run quite high (dark orange to red with API test kit). Screen is about 3x5, roughed up on both sides. Flex of screen keeps it in place on side U-channels.
I was planning to use rigid airline tubing for the air supply. Using the rigid tube for this test setup took a few tries to get right. Very sensitive to angle, hole size, and air flow. After scrapping a few failed attempts that had one end of the tube plugged, I found running the same airline to both ends seems to resolve some of the sensitivity. The Calfo-style return manifold kind of inspired this setup for the airline. Seems like some water gets into the tubing under the holes with a thin layer of air maintained along the top of the tubing. A slug of water still gets pushed around in the center.
A small prototype PCB was fabricated with some generic white leds from the parts bin. There are 30 leds crammed into this area less than a square inch. Rated power is 2.4W (including the current limiting resistors). Lights can be seen in the reflection in the above picture. During initial testing the light was taped right to glass and running at 75% rated power and later about an inch from the glass. At this power output the center of the screen had no growth but there was some coloration visible around the outer edges. Setup ran for about 2 weeks in this configuration without any growth in center. Output was then dropped to 40% of rated power. Photo shows first week of growth at 40%. Lights have been running using a 18 hours on, 4 hours off cycle. (I realized later it would typically be 6 hours off, but used 4 for some reason when I wrote the software for the timer / led driver. Haven't had a need to change the code in the controller yet.)
The PCB plus LED thickness is 0.1 inch. Running the current lights at 75% was pushing the thermal limits. 60°C was measured with a thermocouple on the LEDs. 0805 660nm LEDs on order. Will try 64 LEDs over 4 square inch area next. Hopefully more space between chips will run cooler.
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