Fish Lights -- 60W of Green LEDs

Another Project Post

While I was working on the trailer lights, Dan, the scoutmaster, got talking about using green LEDs to attract fish. He had already made a small light using a 9V battery and a resistor.

Dan says, "Can we make this brighter?" So I upped that a bit and soldered together thirty(?) LEDs on a dowel. I had no way to waterproof it, and that is way too many LEDs to run in parallel but it was pretty bright. 

Not my best soldering job.

A bunch of 30mA green LEDs would around a dowel.

Dan says, "Can we make this brighter?" Next was a series/parallel combination of 24 LEDs on a panel. It was pretty bright. The idea was to put several of these together. It was a pain to do all that soldering on a proto board--I soldered two, only one worked. Dan says, "Can we make this brighter?"

Yep, we can make this brighter. Thus is born the Fish Light. LED Engin makes a small 5W four element green LED. TI makes a handy little eight LED driver (TPS61500) that runs from a car battery. Stick on a simple thermal shutdown from Microchip (TC622) and a simple schematic results. 

Fish Light schematic.

Heat dissipation is a big issue with this design. 10W of power is dissipated by the LEDs plus some more by the driver. I made the bottom layer entirely ground so it could sit directly on an aluminum heat sink. The circuit must be placed on a heat sink to operate. It starts overheating (the light output drops) after about 15 seconds of operation with no heat sink. 

Finished and mostly finished panel.

PCB layout

Since the whole thing is supposed to go under water, I used a Bourns Resettable Fuse for a bit of safety in case of a moisture induced short circuit. I found the LEDs impossible to solder by hand. I had to borrow a hot air solderer to attach them. 

A single panel lit in isolation.
This picture was taken under lights, but the green lights wish all that out.
Without heat sinking, the circuit heats up quickly. 

I made a framework from one inch wide, one sixteenth inch thick aluminum strip. The strips were bent so the LEDs would at a 45 degree angle from vertical, which directs their light out and down but not up. To do that much bending, a bending brake is the tool to use. I don't have a bending brake and didn't want to invest the money or shop space in one, so I faked one up using some scrap steal, C-clamps, and a rubber mallet. Pictures below. I drilled the needed holes first so I wouldn't be trying to maneuver the aluminum strip under the drill press when it was bent into a funky shape. 

Bending brake with Al strip ready for bending

Different angle with the strip bent.

All six panels mounted on the aluminum frame.

Lights on! It takes just over 10A at 13.8V to power get this much light.

This is supposed to work under water. I have no idea how to water-proof electronics. I tried painting the whole assembly (except the LED lenses) with 30-minute epoxy. The problem is the epoxy is clear so I can't tell where I missed. Therefore I did an overcoat of green spray paint, laid on thick. I tested this by putting the whole assemblage under water in the kitchen sink. The first time three of the panels failed, so I dried it all out, reapplied epoxy to those three panels and it worked.

Sitting in the sink, waiting for water.

Under water, lit, same lighting as the last picture.

Things I learned

• I had never used a hot air soldering tool. I have now. I must have improved my soldering in the last few years. I no longer treat an SOIC as a challenge. I did find the HTSSOP package a challenge with its 0.65mm pin spacing leaving only 0.35mm space between pins. 
• I had never used a bending brake. Now I've made a simple one. 

Things I would change

• Those particular LED packages were a royal pain. I could find no way to hand solder them. I had to use the hot air soldering tool. 
• I need to find a better way to waterproof. 

Each panel has $45 of electronics components on it plus the PCB. That's $270 of electronics, about $100 for PCBs, plus the frame. It was worth doing once as a fun project for a friend, it isn't something I'll be making more of. 

Cool project, time to move on to something else!

Bruce McLaren

Dan sent me this picture of the assembly under water in Lewisville Lake. He says, "This has a 12 ft radius in muddy water."