Cheap Simple Bike Light

As an alternative to using the complete Mag-Lite to house the 20 watt halogen bulb, it is possible to make a decent light using just the head of the flashlight. While the complete Mag-Lite version gives the advantage of using the light as a hand held flashlight, the simple version is smaller and clamps on the handlebars better.

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It occurred to me while making the original 'cheap 20 watt lights' that there had to be a simpler method of connection the light to the handlebars and a simpler way of making the light. I then came across a plastic wood clamp (4”supergrip by Rockland Tool) in the hardware section of a local discount store. It was $1. It proved to be the perfect clamp diameter for the bike handlebars. The above photo shows the original clamp on the left and the version with the nose cut off, to allow a better fit with a handlebar bag.

You can use either the “C” or “D” size Mag-Lite. You only need the head assembly. There is a difference in the heads which can been seen in the photos above. I used a “D” size because it was available and not needed anymore. The head is heavier than the “C” and has a different internal configuration. The “D” has a thicker section behind the bulb area that can be tapped with a drill. I was careful to drill just behind an internal flange. This can bee seen in the photos below. The pen caps in the photo above show the relative depth of the flange on the “D” and the shoulder of the “C” heads. The “D” has about 2x the thickness of the “C” in the area where you will drill.

The “D” was.drilled with a 13/64” drill and tapped with a ¼-20 tap. The photo above shows where the hole appears inside the head, just behind the thick internal flange. If the “C” head was used, the hole would start on a more curved section of the outside and come through the thinner wall behind the bulb section.

The plastic wood clamp was drilled to take the bolt. The head of the bolt was just large enough to seat nicely and be prevented from spinning. The bolt was a ¼-20 x 1/2” bolt (seen below in photo) which is just long enough to pass through the clamp handle and through the tapped head. Since you can not turn the bolt, the head has to be threaded on the bolt as in the photo above. You may have to reseat the bolt head several times to get the light to end up tight in the forward direction.

The other parts needed besides the Mag-Lite include a 20 watt halogen spotlight bulb. The GE ESX model (MR16 size) is preferable. This brand has enough internal silver to be used without the extra reflector. If a Sylvania or Philips brand is used, the Mag-Lite reflector should be trimmed and used.

The above photo shows the reflector before and after trimming the rear end and the front lip. It should slide over the bulb as seen in the photo above. The next photo shows how the reflector goes between the light head and the bulb. It will later be assembled with the power cord and screw-on bezel. When the bezel is assembled, keep the clear polycarbonate lens cover in place to protect the glass faceplate of the bulb.

If the bulb used is the GE the slight flashing on the flange of the light will have to be removed. I used a standard grinding wheel, and carefully took off no more that needed to allow the bulb to slide easily into the end of the Mag-Lite head. The above photo shows how little had to be removed. Probably no more that a few thousands. If this is not done the bulb will not sit down properly in the head piece recess. When the light heats up the aluminum will expand away from the pyrex bulb and the bulb may actually slip down and get jammed in the light head recess. (If this happens, just turn on the light and wait until the head gets fairly hot and the bulb will fall out with a little tapping.) When the extra reflector is used the reflector takes up vertical space and the bulb will be above the head recess.

The cord used was a computer power cord, but any lamp cord will do. Two terminals were crimped on the end to attach to the two pins on the bulb. A simple single pole switch was wired and soldered in line with on the the wires. Electrical tape was used to fix it in place on the power cord as seen in the photo above. The auto cigarette lighter plug was attached to the other end of the power cord. The power cord is attached to the clamp with a nylon tie-wrap through a hole in the handle, as seen in the photo below. It should be very tight as this keeps the power cord from unplugging from the bulb pins.

The power supply is the same as the other flashlight, a 12 vdc 7.5 Ahr battery which has the auto cigarette lighter receptacle attached.

The above photos show the method of attachment on the bike handlebars. The clamp is quite strong so the light does not slip, but can be pushed up and down to adjust the beam. If no movement is wanted, put a wrap of electrical tape on the handlebars which the teeth of the clamp will bite into. The last photo shows where the bolt comes through the side wall or the head. After I had adjusted the light beam straight ahead of the bike, I used hot glue to prevent any twisting of the head. To bond the glue better, the light was left on for awhile to get hot and better take the hot glue. Without air movement by the light, it will get quite hot, but because of the open end, it will cool better than the full Mag-Lite versions.