Step 1: Putting Electricity Into the Water
Fortunately LEDs run on low voltage: between 2-4 VDC, this means that even with exposed wires or contacts nearby swimmers won't feel a thing. We just have to make sure we use a power supply thats failure mode wont put 110VAC in the water.
A transformer based power supply will do the trick! To the best of my knowledge these wont go 'live' upon failure. However you may need a large transformer depending on the number of LEDs used.
The best solution i've come across as a power supply (the one i used) is a 12V car/marine battery. This will ensure you will never get more than 12-14V in the water. Be careful though, if shorted these can produce very high current. Disconnect from circuit to charge.
Materials listed in this step:
12V car / marine battery, or deep cycle if possible
Step 2: Building a Current Source for Your LEDs
Well you probably can but you may not want to as it will hurt the life expectancy of your (relatively) expensive power LEDs. We need to make sure these things are operating at or just under their design current.
Initially i thought about using a circuit based on the lm3406 1.5A buck regulator. After calculating the price after making custom PCBs and components I decided on something a little more simple: The LM317 linear regulator.
There are already instructables out there which explain how to use this so I will keep it brief. The 317 maintains a constant voltage of 1.25V between its 'adjust' terminal and its 'output' terminal. If you wire a 1.25 Ohm resistor between the two 1Amp of current will flow (V=IR). Now simply attach your LEDs between adjust and the ground (see the diagram).
Discussion: Although simple, this design is not perfect. The Lm317 dissipates power as heat to control the voltage. If you are supplying it with 40V and using it to drive a 4V LED at 1amp you will be dissipating 36Watts. P=I*V (40V-4V)*1amp =36W. You will want to power it with a voltage very near to that of what you are driving. With a 12V battery and a 1.25 voltage drop across the resistor and 0.5V drop across the IC you will be able to power 2-3 LEDs depending on their voltage
The box shown above consists of 12 identical circuits to provide 0.8A to chains of LEDs
Materials listed in this step
12 - Lm317 regulators
12 - 1W resistors ( value dependent on desired current, I[A]=1.25 [V] /R[Ohm])
12 - cable connectors
1 - on-off switch
1 electrolytic capactitor
Step 3: Wiring and Mounting LEDs
So to run the total of 18 LEDs i used 6 parallel sets of 3 LEDS wired in series. This equates to 3.7[V]*1[A]*18[LEDs] = 66W of juice.
22 gauge telephone cable worked nicely for wiring this. There are 4 wires in the cable, I used one cable to power 2 set of LEDs, (2 wires each) but one could power 3 sets of LEDs per cable while using 1 wire as a common ground, especially if using smaller gauge (thicker) wires.
Hopefully you get the idea by this point and can design whatever setup you like.
It would be nice to make little underwater enclosures for the lights. However due to time and budget I simply mounted the lights on heat sinks (which you will want if running out of the water, they get hot!), the glued the heat sinks together and screwed through the middle into my dock. I used a staple gun to attach and hide the wires along my dock.
Materials used in this step:
18 - Luxeon K2 LEDs $5 each
18 - Luxeon K2 Heat sinks
X many feet of 22 gauge telephone cable (or whatever cable you want)
connectors to connect the wires to your current source.
Step 4: Youre Finished!
Note that it doesn't have to be your dock! My current plans include mounting these on the bottom of my lake to make runway lights for my boat. You can add different lenses to the lights which will collimate or spread the beam. Try different colors too. You can hook them up as long as they are rated for the current your current source supplies (don't worry about voltage).
Step 5: Extending the Project
The first step is to control their brightness. I've attached the design i made for a PWM controlled buck regulator circuit however i later realized that one could just add a transistor in series with the lights and use the PWM signal or an analog out signal from a micro controller to control the brightness of the lights. This is my next step, it can be easily integrated into the finished project.
Changing the colors:
The receptors in your eyes are only sensitive to the red, green and blue wavelengths of light. You interpret a color by the relative excitement of each receptor. For example yellow light would excite the red and green receptors. Thus if we stick red green and blue leds together and control their relative brightness, or the relative amount of time they are on or off with respect to each other (in a very very short time frame (PWM!)) we make our brains think we are seeing different colors. This is how TVs work!
The question is now how do we control 3 different lights (RGB) per group of lights without running a zillion wires everywhere. This is your dock, not your electronics lab. We would need at least 4 wires per set of RGB lights rather than 4 wires for 3 sets of lights as we could do before. The answer is that i don't know how to do this nicely! I'm hoping the informed readership will contribute.
One answer would be to wire all the different colors together with their own colors. ie. all the reds in series and use pulse width modulation to control the relative amounts of that color. This would mean less wires running under your dock but it would also mean that every group of lights on your dock will be the same color at the same time, rather than half green and half purple.
The system could be redesigned completely so that the control electronics are located underwater with the lights. This would require only 2 power wires and one control wire per group of lights. But getting your electronics wet will probably result in failure so this may not be the way to go take.
To sum up the problem: how do we minimize wires, but maintain individual control over the R,G,B LEDs in each group of LEDs? Remember we want to keep any voltages generally below 12 V (can't put all LEDs in series on one string) We are basically balancing degrees of freedom of control with the number of wires. This is a typical example of engineering constraints.
Please send suggestions and any questions you may have.