DIY Bicycle LED Bulb for Reversible Conversion of Old Bicycle Head Lights




Introduction: DIY Bicycle LED Bulb for Reversible Conversion of Old Bicycle Head Lights

LED bicycle lights are nowadays widely spread, there are some nice ones to buy and one can find many instructables on how to build them by oneself. I like the old original head- and rear lights mounted on my vintage bicycles and yet I want to use some of them also by dawn or night. I’ve had battery powered strap-on lights but regularly forget to charge them up! So dynamo powered lights are still the most convenient from my point of view so I liked to convert the old lights to LED. At the same time, I want to keep the old equipment original and don’t want to make irreversible changes to the lights. The only convenient option therefore is to have LED bulbs with standard E10 base which can be screwed in traditional mounts.

There are some LED bulbs to buy which feature the standard E10 base. I first tried a couple of these; some have several SMD LEDs pointing to various directions and some only have one super-bright LED on top.

But these have drawbacks:

  • Most of them are voltage-regulated with a resistor (specified to 6V) and therefore not ideal to maximize the efficiency (I don’t go deeper into LED theory as there are already other awesome instructables on this).
  • The larger disadvantage is that they lack an optical focusing lens or reflector. LEDs spread the light different than incandescent bulbs so the lamp's reflectors don't focus the light to a usable 10° or 15° angle.

I made a LED bulb which directly includes a lens to focus the light, optionally has an integrated rectifier and needs no further electronics to use with a bicycle dynamo.

Step 1: Materials & Tools

Materials used

The LEDs I used in the end are conventional High Power LEDs (available in all colours incl. warm white, neutral white and cold white) with a power rating of 3W (also as 1W, 5W etc).

  • The white 3W LEDs are rated with a maximum current of 700mA @ ca. 4.2V (ca. 2$ for 5pcs)
  • For these LEDs, various lenses are available in internet stores (about 1.20$ for 10pcs). I purchased some for 5°, 8° and 15°.
    Note: 15° is maybe a too small spot for some people, then look for slightly bigger angles. Also, the focus spot will be just a round circle and no bicycle-on-road-optimized square pattern. Works for me but maybe you don’t like it.
  • E10 bulb base to screw
  • Small amount of copper or aluminium plate (ca. 0.5mm or little thicker)
  • 2K epoxy
  • SMD bridge rectifier (for example: model MB6S, 0.5A, SMD, about 1.15$ for 20pcs)
  • Thin wire (I used litz wire but I think a single cable wire would be easier to handle!)


  • Drill
  • File or Dremel or others
  • Soldering iron

Step 2: Prepare E10 Base and Heat Sink to LED

To be able to mount them in traditional lamps, I installed the LED in a E10 bulb base. I took the base from an old broken filament bulb and removed the glass head. The remaining stuff inside is best removed with a drill (6mm). This part is a bit fiddly. The Bottom of the base is drilled with a thin drill (I used 1.0mm).

Two wires are soldered to the LED. The LED is placed on a tiny copper plate which acts as heat sink. This process is a bit fiddly but I don’t think the LED survives without heat sink, which I haven’t tried yet (if you do I very much appreciate feedback in the comments on it). I recommend to glue the LED on with epoxy. Optionally, some heat conducting paste can be placed in between LED and copper strip. Even better: Heat conductive epoxy (which I didn’t have).

From here, two variants to finish the bulb are possible. One is simpler to build but yields less brightness in the end. The other includes a bridge rectifier which is not easy to incorporate into the E10 base.

Step 3: 1. Variant Without Rectifier

Bicycle dynamos are usually rated 6 Volt / 0.5 Ampere, which gives 3 Watt. Because bicycle dynamos are CURRENT sources and not VOLTAGE sources, the maximum output current of them is limited to 500mA. The LED can therefore be attached to the dynamo without further electronic parts in between – and should not burn also at high velocities. The current will be 0.5 A but the dynamo will not be able to deliver more than ca. 4V – the overall power is about 2W which makes less cycling effort. I didn’t mind that the full 3W potential isn’t used because at this current, the LED was already very bright and sufficient to drive in full night! The flickering that you can see (especially at low speeds) is because of the AC source (Dynamo). A LED has a polarity and will, connected directly to AC, only light once in a phase.

The LED on the copper strip is glued directly to the E10 base. One wire (doesn’t matter if + or – ) has to go through the hole in the bottom and soldered into a small ball from outside. The other is soldered to the top edge of the hull. Once the wires are soldered in place, the base is filled with epoxy and the copper is put on top. The copper also has to be soldered to the hull for better heat conductivity. When the epoxy is cured, the overlapping copper is removed with saw, file or Dremel.

In the end, the lens is simply attached onto the LED (clamps strongly enough, otherwise use epoxy). I took the 15° lens.

Step 4: 2. Variant With Integrated Bridge Rectifier

In this variant, a small bridge rectifier is integrated in the E10 base. The bridge rectifier converts the AC from the dynamo in DC. The effect is a less-flickering LED and (ideally) DOUBLE light output! It brings a voltage drop though but as the maximum voltage drop over the LED doesn’t exceed 4.5V at 0.5A, the dynamo voltage (min. 6V) is enough.

Attaching all the wires correctly is quite fiddly and attention has to be taken to create no shortcut! To + and – pins of the rectifier, two wires (red & white) are soldered. To the AC pins, two wires are soldered (blue one ‘down’ for through the hole and yellow one upwards towards LED).

Red and white are shortened and insulation-stripped and soldered to the LED (attention to polarity!).

The copper heat sink is attached to LED (I soldered it to the heat sink but this could harm the LED because of the heat – epoxy might be the better choice).

Step 5:

Before putting the rectifier into the hull, I covered all wires and pins with epoxy so it is well isolated towards the hull.

This is necessary as in the end, only the yellow wire (AC input) is allowed to connect to the hull! Then the rectifier is fitted into the hull. Fill it up with epoxy and attach the heat sink as in Variant 1. The wire coming through the hole in the bottom is shortened and soldered into a ball. The yellow wire (AC input) is soldered to the outside of the hull.

Step 6:

The lens is directly put on the LED (the model I used clamps on quite tight – no extra lock necessary). The finished LED bulb is comparable in size to a standard 2.4W/6V filament bulb and also fits into all bicycle head lights I tried.

Step 7: Light Comparison

A comparison of conventional filament bulb in front light, a as-bought LED bulb without focusing and the built LED bulb with lens. (The pictures were shot with only 4.5V input, at 6V, the lens LED is even brighter in comparison to filament bulb).

Step 8: Rear Light

A conventional filament bulb could still be operated but at the reduced voltage it would light poorly! So for a complete conversion, there are several options. As the rear LED doesn’t have to be high power, no LED with 700mA is necessary to be used.

  • Buy a resistor-regulated LED for here (brightness is far enough also at suboptimal efficiency). Again there are various sorts on the market, some with one LED, some with 5 SMD LEDS etc.
  • Build one in the same style as the front bulb but with a resistor included in the E10 base (see pictures). I used a 20mA super bright red LED, with voltage rating 2.4V (most of red LEDs). This works with a 68Ω resistor put in front. Here also a somewhat higher resistor value can be chosen to be sure to not burn the LED, however, mine has been in service for a long period and still works. The power dissipation is calculated to be ca. 0.09W which is safe with a standard 0.25W resistor. Also, one LED is bright enough and the fact that it runs on AC means less losses for the front light. To improve the light dispersion (at rear we want the opposite to the front, best possible viewing angle), I sanded the LED head which makes it diffuse and improves viewing angle.
  • Integrate a small SMD IC current regulator into the base. The NSI45020AT1G is super tiny and can be soldered in series to the LED like a resistor. It regulates the current to 20mA. However, as it is really small, attaching it can be difficult and I haven’t found it to buy somewhere really cheap.

Step 9: Other Alternatives

If the LED conversion doesn’t have to be reversible but still in proper style, one can directly built the LED including the focusing lens into an old lamp (as already described in other instructables!). I once got a lamp where the bulb holder was missing. I taped a LED directly into the reflector, glued an 8° lens on top and fitted an external rectifier (Picture 1, 2, 3). Looks great AND leads your way home at midnight down a dark mountain (or any other really dark place).

The rectifier can also be built into the lamp separately which has the advantage that the unrectified LED bulb can be used. Such a setup could look like this (note: the bulb holder has to be isolated from the rest of the lamp) (picture 4).

Step 10: Important Comments!

I am no electronic expert and all the things that are to the best knowledge. I have used the bulbs for a while and they still work, so I assume everything is allright but I can give no guarantee on anything electronics-related. I’ll correct it as soon as my first LED vaporizes!

The LED directly mounted without resistor ONLY is safe when the power source is a 6V/3W rated bicycle dynamo and the LED has safely more than 500mA forward current! When using it with a battery, higher currents occur which will damage your LED and rectifier.

Thank you very much for reading this instructable! I hope I could help some of you and inspire some new ideas.

I’m really looking forward to reading your comments and improvement suggestions!

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    2 Discussions


    10 months ago

    Thanks a lot for this article ! I've tried it on my old bike and it works great.

    If I had to do it again, I would buy a larger SMD bridge, because the welding was difficult to do.

    I dit it without heat sink, and the LED survives. I think the metal disc on the bottom of the led dissipates heat sufficiently.

    DIY Hacks and How Tos

    Nice! LED's would be a significant improvement over old incandescents.