Underwater Lights - Insulated in Old Jam Jars




About: I am creating interactive installations and performances. Inviting the spectator to participate in various ways I use any media possible to bring "the idea" to life. Electronics, computers, wood, rope, wat...

At the art ship Illutron we often use lights under water. For illumination and decoration. It is a beautiful effect, and it gives a touch of magic to performances on water.

Leave the lights on for a few hours, and millions of little fish will be attracted and circle the lights.

I recently went to Linz to the art ship Eleonore. I did an underwater light workshop, and that resulted in the images here.

It is really very easy and takes about ½ hour to make

  • Stick a light bulb in a jar
  • Run the cable through the lid using a cable gland
  • Fill the jar with cooking oil and close it
  • Dump it in the water and... Lights!!


  • A jam jar (any jar with a big lid and large enough to hold a light bulb)
  • A cable gland
  • Light bulb with socket
    • Any type is fine. LED, compact fluorescent or classic. But you want as much power as you can get.
  • Cooking oil


  • A step drill (unibit)


  • How to mount light bulb sockets and plugs on a cable

Step 1: Drill a Hole and Mount a Cable Gland

Drill a hole in the lid using a step drill / unibit.

If you don't have a step drill, buy one. You can use other types of drill to make the hole, but because the metal of the lid are so thin, it is not easy to make a nice hole without the metal ripping.

Smooth the edges of the hole with a file.

Mount the cable gland

Stick it through the hole in the lid and tighten the plastic nut on the back side.

If you, like us, did a poor job drilling a nice hole you might want to add a rubber gasket on each side of the lid to make a better seal.

A piece of old bicycle tube makes an excellent gasket

Electricity and water

Around now, you are probably wondering if it is safe to use 230 V electricity under water. As long as everything is properly sealed and insulated from getting in touch with water, all is good.

Getting a jam jar and a cable gland water tight against pressure can be difficult. If we just had air in the jar, water would easily be able to squeeze in and flood the jar because air is compressible.

We fill the jar with cooking oil (cooking oil is a very fine insulator) to make it impossible for any water to squeeze in. Cooking oil is incompressible, and, since we place the jam jar upside down, should a little drop of water come in, it will stay around the gland and not get up to the electric wires, because oil is less dense than water.

  • If you don't feel confident with using 230 V this method works just as fine for 12 V bulbs.
  • Make sure your power outlet is protected by a

    GFCI (Ground Fault Circuit Interrupter). In some contries, a GFCI are mandatory on all installations, but if this is not the case where you live, consider getting an inline GFCI. Thanks

    user Scarecreaux for pointing this out to me.

Step 2: Mount the Socket and a Plug and Fill the Jar With Cooking Oil

Run a piece of cable through the cable gland.

  • Use at least 20 meters of cable, to have more freedom of placement.
  • If in doubt, use a longer cable. You can always make it shorter.

Mount a light bulb socket on the inside, and a plug on the outside.

  • Do this as you normally would do.
  • If you don't know how to mount sockets and plugs, get someone to show you. It's easy.

Fill it and close it up

  • Fill the jar with cooking oil
  • Close the lid
  • Tighten the cable gland around the cable.

We use cooking oil, but any oil could do. Cooking oil is just more eco-friendly and harmless to the environment, should you spill a drop.

Mount the jar upside down

Make sure you get the jam jar hanging upside down. We just used a couple of zip-ties around the jar.

We want the jam jar upside down to keep the oil inside, if the seal at the lid or cable glad leaks a little. Oil floats on water and will stay at the top and not escape.

Step 3: Test the Light and Dump It in the Sea

Make sure all is working.

I suggest trying the lights in a bucket first. This will also rinse of any excess oil.

Then find some nice places around your boat or whatever you have on water to place the lights.

Don't sink them too deep. 1-2 meter is fine. The water dims the light a lot, and the lights will look quite puny at 8-10 meters depth.


There are a lot of possibilities for variations and combinations. RGB-leds with color control. Placing many lights over a larger area etc.

All these experiments are easily done, because this method is so quick to assemble and reassemble.


Most of the photos here where taken by Rosa Angora during a workshop I did at the art ship Eleonore in Linz, Austria in late 2014.

Photos from Illutron in Copenhagen are by Mathias Vejerslev

Step 4: Update: Halogen Tube in an Old PET Bottle

If you want immense power on the light, you need something like a halogen tube.

Trouble with halogen tubes is, that they burn so bright, that they easily overheat and die.

And, handling is difficult, if you touch them, the sodium from the salt-sweat on your hands will destroy the crystal glass at temperature. Same goes for salt water, obviously.

But, the oily trick works for these too!

  • Solder wire to the ends of the tube
  • Stick the tube in an old plastic bottle
  • Run the cable through the cap using a cable gland
  • Fill with cooking oil

Will the solder not melt in the heat?

  • Yes it will! - and that's good.
    • If the oil for some reason does not cool properly, the tube will heat up, the solder will melt, and the wire will fall of. Built in thermal fuse.

So far I have tried a 250 W tube in a 500 mL bottle. That worked fine.

A 1000 W tube in a 1000 mL bottle overheated. Probably needs a 1.5 L bottle to have enough circulation and cooling inside from the oil.



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    19 Discussions


    3 years ago

    I seen computer builds with mineral oil. Does it make any different using mineral oil than just regular cooking oil?


    4 years ago on Step 4

    Great idea. I would not expect the halogen bulb lifetime to correspond to its normal life span, as a main part of the function of a halogen bulb is that it has to be really hot to last. The glowing wire in a halogen bulb evaporates due to the very white/warm glowing (high temperature on the wire). It solidifies on the glass, but beacause of the halogen gas and the very warm glass, the metal is returned to the glowing wire. When you cool the glass, it will go silver coloured/black faster, and the wire will get less metal back.

    2 replies

    Reply 4 years ago on Step 4

    That's interesting! - I will inspect one of the lamps, that we have been having in the water for about a month now, to see if there is any unusual deposits on the tube.


    Reply 4 years ago on Introduction

    I looked at the longest living lamp just before I left. There was no visible deposits on the tube. Thanks for coming by the workshop today :-)


    Reply 4 years ago on Introduction

    The light definitely attracts both smaller and bigger fish. But using light to attract fish to catch. could easily be illegal where you live, so check your local regulations first.


    Reply 4 years ago on Introduction

    We do get a bit of yellow tint from the cooking oil. But we chose to stay with the cooking oil for the environmental aspects. I would actually imagine, that clear food-grade oils exist, but they are most likely more expensive than the 20 l drums of sunflower oil I can get at the megastore :-)


    Reply 4 years ago on Introduction

    The oil is there to make sure the water stays out, in case the lid or cable gland leaks. Filling things with oil in wet environments are quite common.
    It is my experience (from many years of work with electronics on water) that making water tight seals for submersible enclosures is not an easy thing.
    Neither jam jars, nor cable glands are designed to withstand water under pressure from the outside. Proper solutions for this exist, of course, but then the price tag of this build would soar.


    4 years ago on Introduction

    Might I suggest using an in line GFCI, the type used often on construction sites which flexible power cords plug into. When used properly at the source this would protect from electrocution should something not go as planned. The link below is just to give an idea of the type I am referring to.


    1 reply

    Reply 4 years ago on Introduction

    Very good point. I was not aware, that GFCI (Ground Fault Circuit Interrupter) was available for inline mounting.
    Having a GFCI on the line supplying the lights are a must.
    Here in Denmark, GFCI are mandatory on everything, so it would be almost impossible to come across an unprotected outlet, but I realize, that this is not the case everywhere. I will include this in the instructable. :-)


    4 years ago

    This looks neat and the instructable is nicely done. My only concern is that this should not be used in open sea water. The lights will cause problems for wildlife and may even be illegal in some areas. Anyone who has been to a seaside resort may have noticed the odd light fixtures along the beachside of the resort. They are designed to let as little light as possible be directed to the sea.

    1 reply

    You are very right on this. I expect of people to adhere to local rules and regulations.

    Here in the Copenhagen harbour, the lights are only visible for 20-30 meters max because of the murky water.

    My main aim with this instructable is to show a very inexpensive DIY alternative to the commercially available underwater lights.


    4 years ago

    Pool lights would be beautiful. If you put two cable glands in the lid of the jar you could easily daisy chain several lights.


    4 years ago

    Thanks for sharing this. I'm inspired to build something for my dad's pool this summer.