Introduction: Bayonet Plug Compatible LED Bulb for Older Sailboat

Do you have an old sailboat, trailer, RV or off-grid cabin that uses bayonet-style (12 V) light bulbs?

Are you worried about running down your battery bank, and not being able to start your engine when it's time to move on?

LED bulbs draw very little power compared to older incandescent bulbs, so changing over to them can help increase the "life" of your battery bank between charging sessions.

I chose to convert my old bulbs because they were handy, they were definitely the correct for the light fixture I was working on, and it was cost-effective.

Step 1: List of Materials

LED light assembly (like the excellent ones from the good folks at Bebi Electronics - properly engineered for marine service and carefully manufactured - see ) Note that some models of LED assembly are brighter, and some have been designed to produce a warmer colour light.

Old bayonet-style incandescent light bulb (to be cannibalized for the base) - Note: to ensure you have the proper style (i.e. 1 or 2 contacts on the base, and the shape & location/orientation of the bayonet studs, I suggest using one of your existing bulbs - you won't need it any more!)

Before you start this project it's a good idea to go out to your boat/trailer/RV and (carefully) stick your multimeter probes into the light fixture you want to change over and test which contact has positive polarity. (On my boat, different fixtures took different styles of bayonet bulbs, so be careful with this step.)



  • Multimeter & probes & test leads with alligator clips
  • 12 VDC source (e.g. a car battery!) for testing before final assembly
  • Soldering iron & solder
  • Heat shrink tubing & heat gun
  • Metal tape
  • Silicone sealant & caulking gun
  • Pliers and masking tape
  • Gloves and safety glasses
  • Closed cell foam sheet (to make optional spacer, depending on the nature of your fixture)

Step 2: Getting Ready for Assembly

The first thing to do is to carefully remove the glass part of the bulb. (Wear gloves and safety glasses.)

Before I used the pliers, I first scored the glass with the edge of a sharpening stone. One way to reduce the risk is to wrap masking tape around the glass before you use pliers to break the glass and remove most of it. (You can leave some around the edge, as it helps support the shape of the finished LED bulb.)

Do not break the central glass support for the filament. Remove the filament but be sure to leave the wires extending out the top of the glass support, as you will be soldering the leads (insulated wires) from the LED assembly to them.

Step 3: First Stage of Assembly - and Testing

The idea here is to solder the leads from the LED assembly to the wires inside the old incandescent bulb - as shown in the first photo.

Polarity of the wires is CRUCIAL as the LEDs work only if the red wire goes to the positive contact on the bayonet base.

Some bases have two grey contacts on the bottom, and some have only one in which case the brass acts as the other. It's not obvious which wire coming out of the top of the glass goes to which contact on the base, so you have to use the multimeter (and measure the resistance as shown in the second photo).

The best way to be sure which contact acts as positive is to (carefully) stick your multimeter probes into the light fixture on your boat/trailer/RV (before you start this project!)

Once I had the leads soldered, I tested the connection and polarity with a 12 VDC source (e.g. a car battery) and test leads with alligator clips.

I put heat shrinkable tubing on the soldered connections and used a heat gun (or torch) to shrink it.

Step 4: Final Assembly

The next step is to build up the support structure and give the new light its bulb-like shape (so that you can push it into the base and twist to seat it).

I used metal tape to form a cone, with silicone caulking filling in the space.

Leave the metal tape open a little until the silicone cures, then close it up nicely as shown in the last photo.

Step 5: Finished Installation!

For one of the light fixtures I made a little doughnut-shaped spacer out of white foam to improve the final appearance.

If the bulb doesn't work when you first try it, it might be that the polarity is wrong, in which case you will have to resolder the red and white LED wires to the other posts.

After you have upgraded your lights to LED, you can turn them on and read in comfort - without having to worry they are drawing down your precious battery bank!

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