My motorcycle's licence-plate (type 1156) illumination bulb recently failed and rather than buy another incandescent bulb, I decided to hack/repair/convert it to LED. This LED hack should last much longer that the incandescent bulb that it replaced.
Caveats: This hack worked great for my license plate illumination, but it will not work for brake light application since these are usually dual filament bulbs and it will not work for turn signal bulbs unless you also change your flasher to one that is compatible with LEDs. There are many sites that sell LED retrofit bulbs and flashers for older vehicles for some examples, search Ebay for "LED 1156" or "LED 1157" or "LED 3056" or "LED 3057" or "LED flasher".
Parts required: 4 white LEDs
Note: For my build, I used LEDs salvaged from another project, and most white LEDs should work or this license plate illumination hack, but before you purchase your LEDs you should check the brightness rating on the data sheet and verify that it is adequate for your application e.g. Digikey part # 160-1850-ND has a brightness rating of 17000mcd.
Tools required: gloves, safety glasses, pliers, wire cutters, soldering iron, hot-glue gun, multimeter, power supply (for testing)
Step 1: Deconstruct the Bulb
Do this step inside a trash bin so glass will be contained and wear gloves and wear safety glasses for protection. Use a pair of pliers to remove the glass bulb. You will also need to remove the inner glass structure to expose the 2 internal wires. Use wire cutters to trim the 2 wires to a length that extend about 1/8" out of the base (see picture). Use a multimeter to determine which of the 2 wires is ground and make a note of this for the next step (wiring the 4 diodes in series).
Step 2: Wire Four White LEDs in Series
Wire the four LEDs in series as shown in the circuit. The LED anode(+) and cathode(-) can be recognized by the flat side of the plastic lens or by the lead length as shown in the diagram. My application was for licence plate illumination so I wired the LEDs in a straight line and oriented them in the direction that I needed the light. Unlike incandescent bulbs, LEDs are a directional light source so arrange your LEDs appropriate for your application.
Once you have everything soldered, apply +14V to the bulb. The preferred method to test this is with a current-limited bench power supply. If it doesn't light up, check your connections and polarity.
Step 3: Encapsulate the LEDs With Hot Glue
Apply hot glue to fix everything in place. You could also use epoxy, but hot glue is faster and should be adequate for this application. Make sure you have the LED orientation correct before you glue it and also make sure no wires touch that could cause a short.
After the glue has cured, it's ready for the final test. Plug the new converted LED into the socket and verify that it works.