In the wintertime the evenings get dark and therefore plunging in to the dark lake is not as pleasant as it is in the summer. Led lights to the rescue!

Above is a photo of the lights in a pool, or actually a hole in the ice in a lake.

Submergible led lights can be made with a car battery, a couple of 1 watt leds, a constant current driver, a piece of clear 1 inch diameter PVC hose and two small pieces of e.g. aluminum profile for led heatsinks. The leds go inside the hose together with some kind of weight that will make that part of the hose to sink (or you could use an external weight as well). You just need to fold both ends of the hose to make it watertight. Below is an image of the setup that I used. The hose is attached to a couple of pieces of scrap wood to hold it in the position that I want it to be in.

I used a brass bar as the weight (just because I had one that I did not need). The two 1W white leds are attached to pieces of L shaped aluminum profile that act as heatsinks (below). I also added a piece of steel strap just to keep the led parallel to the hose. The green string in the photo is just something that I used to pull the led system into the tube.

Constant current driver

The constant current driver could be separate from the hose and the leds, but I decided to place it inside the hose, to the far end, just to have a waterproof casing. Only the power cord (to be attached to a 12V battery) comes out of the hose.

The driver is a standard, inexpensive LM317 regulator operating in constant current regulation mode as described in the LM317 datasheet. The only parts needed are the LM317 and a resistor, 3.6 ohms. According to the datasheet the schematic below will give a constant current of 1.25 / 3.6 = 0.347 A. The resistor needs to be able to take at least 0.5 watts. This is not an optimal driver as both the regulator and the resistor will waste as heat a lot of the power that the circuit consumes. The wasted energy is roughly equal to the energy needed for one of the leds. But this is still adequate for my purpose. A medium size, 60Ah, car battery will keep the lights going for a week.

The PCB is equally simple. In addition to the parts above, it has terminal blocks for two leds in series and one for the power input.

The leds are 1W warm white star leds, no brand name, from eBay from China. According to the seller, forward voltage is 3.2 to 3.4 volts and maximum current is 350 mA. Supposedly 90 to 100 lm, but I would not believe that. Price was only 35 euro cents per piece, in a batch of 10, including postage.

Parts list