This is a quick instructable on how to convert a cheap LED flashlight into a headlamp. The Flashlight being used is one you can commonly find in the checkout aisle of hardware stores or sold in bulk blister packs for cheap. They are quite cheap (both quality and cost wise), but this re-work of them makes them much more reliable and useful. Look for the "9 LED Flashlight" powered by 3xAAA batteries. They have a simple aluminum tube for a body, with a clicky switch in one end cap and 9 LEDs behind a plastic lens at the other. The 6LED variety that looks similar should work too, it just will not be as bright. They sell for $3-$5 each, and are barely worth it on their own. The bulk pack I got had 6 for ~$9, including batteries, of which all were corroding badly and quite dead. So far I have made 3 of these, refining the process and design each rev.
Step 1: Needed Stuff
- LED flashlight as shown
- 1x 18650 Lithium Ion batter, or 3xAAA and the adapter that came with the light
- Soldering Iron, flux, solder
- wire tool (for cutting and stripping the wire)
- screwdriver, dental pick, other long narrow tool for pushing the LED driver puck out.
- heat-shrink tubing to fit the wire size used
- 20-24awg gauge wire, ~2' worth (I used an old computer ribbon cable, not recommended, breaks easy)
- Clear 2part epoxy
- Something to mix the epoxy on/in/with, and apply it
- latex/vinyl gloves (keeps the epoxy off your hands)
- Spade connectors (or other 2pole power coupler)
- Spray adhesive
- small piece of neoprene or other foam padding
- Small piece of thin flat sheet metal (aluminum/tin/steel can side, as long as its not ribbed)
- small copper strip or battery contact spring
- Drill and bits (1/8")
- amc7135 current regulator chip (available on power regulation boards sold on dx.com and other similar sites), or ~30 Ohm resistor
Step 2: Disassembly
First, we need to take the light apart. Unscrew the cap and remove the battery holder. Using a screwdriver or pick, pry out the plastic "lens" covering the LEDs at the other end. It is normally just press-fit into the body. Continue by prying out the plastic reflector. Sometimes a wire spring clip is used to retain it, if so pull that out first. You might be able to push the whole lamp assembly out with a screwdriver or other object by simple pushing up through the battery area. If the LED assembly still has the reflector attached, remove it. You should now have a few pieces: Main body tube, battery end cap w/clicky switch, LED board, reflector. The lens can be tossed.
Step 3: LED Unit Assembly
The LED board is the main component of this project. It has 9LEDs setup to run at ~4.5v (3xAAA@1.5v), which we will be running with a single 18650 Lithium Ion cell (~3.7V) and optional regulator. The first one I made did not have a regulator, and while it worked OK for a few hours, the LEDs started to wink out as they overheated or pulled too much current. The AMC7135 I used limits current to ~350mA. With 9 LEDs that is 350/9 =~ 39mA each, which is still a bit on the high side for 5mm white LEDs. An additional small resistor on the output pin might help, or (my next version) just use a ~30 Ohm resistor in series with the board. Since this will be a headlamp, we also need to attach a back plate to the board so it can be attached to a head strap or a helmet.
- Remove the spring from the LED board. You can de-solder it or simply snip it off flush with the board.
- (optional) File/sand the LEDs to the desired thickness or to adjust the light pattern. I used sandpaper on an electric drill to speed this up. Sand the LEDs down to the height you want, but be careful to leave plenty of room above the junction (the metal bits you can see inside). After sanding coarsely, go over it again with fine sand paper, and finish with a polishing compound (blue, for plastics).
- Sand/file the soldered side to reduce the tails of the LEDs poking through. These are cheaply made and sometimes the tails are extremely long. Be sure to stop with enough of a solder point left. My first try resulted in a few cracked joints I had to touch up with the iron
- (recommended) Solder on the regulator and/or resistor. The chip is tiny, and the solder points are very close together, so use a fine-tipped soldering iron and test with a multimeter. See the image as to where to attach it. The regulator has 3 contacts: Vdd, Gnd, Out. Vdd connects to the + of the battery, which is also the + of the LED board, usually the center contact where the spring was. Out is the controlled output, which works by controlling the connection to Gnd. It connects to LED -, usually the outer ring on the board. The center pin is Gnd, and connects to Battery - . Be absolutely sure none of the contacts are shorted, as this will either short the battery (fire danger!!) burn out the regulator or cause it to be bypassed.
- Solder a Lead wire to the center contact for battery +, and one for battery - . These wires can be short, just long enough to get off the board and have room to work. Longer wires can then be soldered into place with the junction covered in heat-shrink tubing.
- Let the solder joints cool for a minute while you mix some epoxy, then coat the back of the chip in ~1mm of it. I used 5minute clear epoxy. Make sure the Lead wires are laying against the board (and make sure you know which is which, + - ).
- Fill in the front with clear epoxy. This will prevent water from reaching the contacts, help hold the LEDs in place (instead of just the thin solder joints) and keeps mud/dirt/debris from getting stuck in it. Wrap painters tape around the edge of the board to act as a form, then pour in the epoxy. Tease out any large bubbles using a tooth pick. I usually pour in enough to fill just below the tips of the LEDs.
- Let the epoxy cure.
- Sand the back with coarse sandpaper lightly to give it some texture and flatten any major bumps, but make sure no metal contacts/leads are exposed.
- Make a metal backing plate by cutting a strip of thin sheet metal (aluminum or steel can side works great, I cut mine from an empty SeaFoam can). It should be ~1.5"x0.5" with rounded corners, just long and wide enough to drill a ~1/8" hole for a screw in each end that sticks out from under the LED driver.
- Dry-fit the metal backing plate to the LED board. Check the plate against the leads for any shorts and correct them.
- Using more epoxy (I used JB Weld this time) glue the metal mounting plate to the back of the LED board.
- Let the epoxy cure and the lamp is mostly done. You can use spray-adhesive to glue on a patch of neoprene or fabric if you want, this will make it more comfortable if you wear it on your head as opposed to a helmet, and helps give a better mount when bolted directly to a helmet.
Step 4: Make a Battery Pack
The tube of the flashlight fits the 3xAAA battery holder, which would work just fine for the end result. The tube can also hold an 18650 Lithium ION cell if modified slightly, which will power this lamp for quite a few hours, and is rechargeable. I converted one of the AAA holders to fit mine too, so I could use standard batteries in a pinch.
- Take a small strip of copper, ~3/4"-1" x 1/4" and bend it in half to make a V for a battery contact. Bend a small bit of the end of one leg of the V down slightly toward the other leg. Another option is to re-use the spring removed from the LED driver, though it is harder to solder and fasten in place.
- Take the plastic reflector and fit the bent strip into the more recessed side, flat leg against the back of the reflector. Put the lead wire through one of the convenient holes near an edge. The highest point of the copper contact should be at or very close to the center of the reflector.
- Solder another small strip of thin copper (~1/4"x1/2" or so) to the other (-) lead. This strip will contact the tube to provide Battery-.
- Fit the reflector with the + contact installed into the non-threaded end of the battery case and put in a battery. Screw on the end cap completely and adjust the bend in the contact until the reflector fits flush to the end.
- Once things are adjusted and a test fitting sits flush, remove the reflector assembly from the tube.
- Use epoxy to secure the + contact in place. Use enough to cover the entire inside bottom of the reflector and cover the flat leg of the contact.
- With the epoxy on the inside still curing, coat the other side, filling all the LED holes and shallow recess to the lip. Be a little generous and fill the back side, as this will be the outside of the battery pack when done.
- Place the reflector, back side down, on wax paper to make the epoxy cure flat.The lead wires should be run to the side as this will be their final position when the epoxy cures. Press down and hold this assembly via the battery contact to make sure everything remains in place. Hold it in this manner until the epoxy cures enough that it will not move.
- Let the epoxy cure.
- Slide the resulting puck into the non-threaded end of the tube and see if it fits the same as before. Remove it and try again with the - contact on the side of the reflector, so that it will be sandwiched between reflector and tube. Bend/file the contact and reflector until it can slide in, but keep it a tight fit.
- Coat the outside edge of the reflector assembly with epoxy, then place the - contact where it fit and slide the assembly into place in the tube. Make sure epoxy does NOT get between the contact and the tube. Try to get the reflector assembly as flush as it was earlier. Add in a battery and close the end cap to make sure everything fits as it should. Double check that the - contact lead actually connects to the tube itself using a multimeter
- Let the epoxy cure
- Clean up any epoxy drippings, check continuity of the - and + leads one more time (double check there are not shorts), then add in a battery and check that you get ~3.7V from it. Test that the clicky switch still works. If its all good, you are done!
*Note, as I mentioned, an 18650 is a bit longer than the original 3xAAA battery holder. To use the 3xAAA adapter only, fit that in the tube instead of an 18650. To use both, extend the - contact of the adapter until the length is the same as an 18650 cell or get a longer adapter.
Step 5: Power Couplings
This is optional, but recommended. You could have the LED head unit attach directly to the battery pack with a continuous wire. This works fine but limits options and flexibility, so I connect the two with a power coupling. For these smaller lights, I use simple crimp-on spade connectors, but solder them on the wires instead of crimping for a more secure connection. To keep polarity correct (LEDs only work in one direction) I use male on one lead, female on the other. The battery pack leads are setup to match. On my larger lights I use SAE 2pole 12v connectors. Either will work fine.
- Strip the ends of the leads, both battery and LED.
- Arrange the connectors so the proper poles can connect to each other without being able to mix + and -.(+ = (LED)male-(Battery)female, - = (LED)female-(Battery)male. SAE and others, follow the color codes of the wires, red=+, white/black=- for both sides of any one connector pair. *IF you have other lights like these, make sure you setup the polarity in the same way so they can be used interchangeably!*
- Cut 4 short ~1" segments of heat-shrink tubing that fit snug over the individual leads.
- Slide the heat shrink tubing as far from the end as your leads allow.
- If using spade connectors, use the insulated type, but for now pull the metal connector out of the insulating hood with needle nose pliers. Slide this insulator down the lead wire as well.
- If using SAE or similar pre-made connectors, solder the connectors to the leads. For Spade connectors, flux the wire ends, insert and crimp as you normally would, then solder the wires into place, filing the crimp with solder. Let the junctions cool.
- If spade connectors were used, slide the insulating hoods back into place, making sure they slide completely into place (use needle-nose pliers again to help pull the hoods on.
- slide the heat shrink over the soldered wires, or up into the insulating hood as far as it will go, then heat it to shrink it into place.
- Once everything is together, connect the connectors and test the light!
Step 6: Mounting
I bolted mine directly to my helmet. I drilled small holes in my helmet (!! do this at your own risk, it can weaken your helmet and invalidate any certification/warranty it might have!!) where I wanted the lights mounted, then used computer case screws to mount them directly. Another option is to attach the light to a strip of elastic and mount/wear like a standard headlamp.
To mount the battery pack, I have tried a couple methods. One used a couple large (~1") plastic cable clamps bolted to the back of the helmet. These were large enough to hold the battery pack created here, but made mounting more than one difficult. Since I use multiple lights on my helmet, I ended up using a copper pipe strap and bending and bolting it to the back of the helmet to fit.
The wires I ran under the lip of the helmet and under the velcro'd lining of the headband. My helmet is a Petzl Ecrin Roc, which has a replaceable lining on the headband that mounts via velcro. I undid part of this lining, tucked the flat ribbon cables into it, then velcro'd it back into place. At the back of the helmet, the wires go out of the lining and under the lip again to meet up with the battery packs. Some helmets have built-in mounting brackets that could also be used if you can make a mount to fit them.