Maglite 2AA Conversion to a White LED

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Introduction: Maglite 2AA Conversion to a White LED

This project retrofits the incandescent bulb version of the Maglite 2AA flashlight with a white LED.

The 2-AA Mini-Maglite's official specification are:

14 Lumens total light output

2305 mcd maximum beam intensity

5.25 hour operating time

There is a lot of controversy on the actual light output of the Maglite 2AA. The bulb darkens over time, which decreases the light output. Additionally, the light output falls rapidly, while the light’s color temperature shifts to the red, as the battery drains down. Figure 1 shows the darkening of the bulb. Figure 2 shows the bulb current and power vs. the battery voltage. The opinion of several flashlight forums is that a 4 to 5 lumen specification is more realistic.

While there are LED replacements for Maglite 2AA, they concentrate on increasing the light intensity output to 30 to 175 lumens. At the lower lumen output the battery life may be increased by 2 to 3 times while at the higher lumen outputs the battery life is reduced

The goal for this project is a LED conversion that will maximize battery life with a LED retrofit that has a light output close to the 4 to 5 lumen level.

There are several low cost white LEDs and LED driver chips are
available. The driver chips are switching regulators design to provide a constant current to the LED, which offers the benefit of a constant light output independent of the battery voltage. An additional benefit is that LEDs are a more efficient light source and the battery life will be significantly longer for the same light output.

Supplies

Tools that are required:

Fine tip soldering iron

solder paste or liquid flux and 30GA solder

tweezers

pliers and wire cutters

PCB holder

Voltmeter

Magnifying glass or headset

surface mount component soldering skill

Bill of Material

See the file: BOM_Maglite_2AA_Conversion.pdf

Step 1: LED Selection

Figure 3 shows a disassembled Maglite 2AA.

To use the existing focus control of the flashlight, the LED will
need a wide angle output and its beam intensity will have to be in the range of 2300 mcd. or more. A second criteria I used is no modifications to the components of the flashlight other then replacing the incandescent bulb.

Figure 4 is a picture of the hole, in the back of the reflector, for the bulb. Its diameter is 0.128” (3.25mm), which limits the diameter of the LED to a T1 size (a 3mm diameter). After reviewing the selection of T1 white LEDs available from several distributors the Vishay part number VLHW4100 was chosen because the combination of 7150 mcd Luminous Intensity and a viewing angle of 45 degrees provided the greatest total light output of about 3.5 lumens. Figures 5 and 6 are dimensional drawings of the bulb and the LED. The LED’s maximum diameter is smaller then the diameter of hole in the reflector. No modifications of the reflector are required.

The specifications for constant current boost regulators, for driving white LEDs, were evaluated. The CAT4137TD, from ON Semiconductor, was selected. It is designed to drive LEDs with a constant current up to 30mA from a 2 to 5 volt supply and is available in a SOT23-5 package.

Step 2: PCB Design

PCB Considerations:

The bulb socket and switch assembly in this flashlight is 0.560 inches diameter. This determines the size of the PCB. Figure 7 is a picture of the switch plate with dimensions. The PCB will have two pins that connect to the battery power through the switch plate on the same side as the LED driver circuit, while the white LED is mounted on the opposite side of the PCB from the power pins. The PCB is assembled into the flashlight assembly between the pressure switch plate and the reflector.

Figures 8 and 9 are the schematic and PCB image for this circuit. The Eagle Cad 7.7 schematic and PCB files are ML_2AA_Conversion_02.sch and ML_2AA_Conversion_02.brd. The PCB can be purchased through www.oshpark.com under the project title ML_2AA_Conversion_02.

While the PCB design is a 2 layer board, it can be fabricated as a single layer board and assembled with the installation of the wire jumper J1.

Step 3: Assembling the PCB

Figure 10 shows the PCB and the SMD components that will be soldered

to it. Figure 11 shows solder paste applied to the SMD pads, and Figure 12 shows the components soldered to the PCB.

The two pieces of uninsulated 24 gauge wire, 0.410” long, are made as follows: Start with a wire, 3 to 4 inches long. Grip about ½ inch on each end with pliers. Pull the pliers in opposite directions until the wire breaks. This will straighten and work hardens the wire, which will used for the leads to the flash light's pressure switch. Cut two pieces 0.410 inches long.

On the component side of the PCB, insert the wires into the two smaller diameter holes in the center on the PCB and solder in place. Figure 13 shows the wires soldered to the PCB.

The white LED is soldered to the PCB on the side opposite the other components. The LED's placement is 0.125 inch above the PBC's surface. This places the LED's emitter at the correct position in the reflector to approximately maintain the original range of focus.

Cut a piece of light weight card board, such as a business card, that is about 1/2 inch long and 0.125 inch wide. Place the LED's leads in the PCB's holes. The cathode of the LED, the lead on the side with the flat on the LED's package, aligns with the flat side of the LED outline on the PCB. The cathode is also the short lead lead of the LED. Use the 0.125 inch wide spacer to position the bottom of the LED’s body the correct distance and the PCB. see Figure 14. Solder the LED in place and trim the LED's leads on the component side.

Clean the PCB. If water soluble flux was used then wash PCB assembly with water, otherwise wash PCB with alcohol to remove flux and solder paste residue. Gently, dry the PCB. This can be done with a heat gun using the low temperature setting.

Step 4: Assembling the Flashlight

Determine the polarity of the bulb contacts in the switch plate. With batteries installed measure the voltage at the bulb’s socket in the switch plate. Place a tag, or other marking, to identify the positive terminal. See Figures 15 and 16.

Remove the batteries from the body of the flashlight. Place the leads through the nylon spacer. See Figure 17. The wire lead that is adjacent to the inductor is the positive lead. Insert the wire leads from the PCB into the flashlight's pressure switch terminals. Observe the correct polarity and ensure the spacer seats in the recess of the switch plate. See Figure 18.

Install the batteries and end cap. The LED will light at this point. See Figure 19. Reassemble the focus control, reflector and lens cover and reattach to the flashlight. Do not over tighten the focus control when turning off the flashlight. The nylon spacer centers the force on the pressure switch and protects the components from contact with the switch plate

Step 5: Some Performance Measurements:

The boost regulator constant current LED driver is a constant power circuit. The power consumed by the LED and switching regulator components will be approximately constant over the batteries’ operating voltage range. Figure 20 plots the power supplied by the battery and the power used by the LED. Figure 21 plots the current drawn from the battery and the current through the LED as a function of battery voltage. The LED current is constant within a couple of mA over the batteries’ voltage range of 1.9 to 3.2 volts. The switching regulator has an under-voltage lockout circuit that shuts the regulator down when the battery voltage is at or below about 1.8 volts.

The approximate average current for this LED driver is 41mA over the batteries’ voltage range. A survey of various alkaline AA cells mAHr capacities, and their discharge curves, list capacities ranging from 2100 mAHr to 2600 mAHr. A rough estimate for the operating time of this LED circuit would be from 52 to 63 hours on one pair of batteries.

Step 6:

Here are the kicad files.

The .txt extension will have to be removed from the ...KiCad.pro.txt file.

The .brd extension will have to be removed from the ...KiCad.kicad_pcb.brd

Instructables did not permit uploading files with a .pro or .kicad_pcb extension

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    50 Comments

    0
    dialup_prisoner
    dialup_prisoner

    8 months ago

    It turns out the Maglite I had in mind for my conversion is the 2x AAA version. The switch plate on this one is a mere 10.6mm in diameter but it looks like the components can be made to fit if we go down to 0603 size for R2 and C2, and add some more double-sided tracks. This is probably not a PCB you want to etch at home! A smaller inductor would help, but reducing its size would probably decrease efficiency. I've modified the PCB (thanks again to author, Waltx, for the Kicad conversion!) and placed an order at JLCPCB... hopefully in a couple of weeks we can see if my modifications have worked! If they do work, I'll share the modified files on here...

    0
    waltx
    waltx

    Reply 8 months ago

    Thanks for the update. I also have a maglite 2AAA that I was planning to convert to an LED. And you are correct that the components sizes would have to shrink several sizes. Let me know how your design works.

    0
    dialup_prisoner
    dialup_prisoner

    Reply 7 months ago

    It works! I might increase R1 a bit more to bring the brightness down - with a PCB this small you can actually feel the heat. It does seem stable though. I've attached all of your files that I have changed. You'll need to remove the .txt extension, then unzip. Let me know if it doesn't work.

    0
    barryc61
    barryc61

    8 months ago

    Hi, Great project. I have the PCB. However, please advise where the 22uH inductor and the 1uF 25v capacitor can be sourced from. Unfortunately it is difficult - if not impossible - to purchase low quantities of SMT components, in which case it would be extremely helpful if sources were declared. Many thanks in anticipation. Kind regards, Barry

    0
    waltx
    waltx

    Reply 8 months ago

    Hi:
    A while back I purchased all the components from digikey.com. Today I see that the inductor is not stocked at digikey, but they do have the capacitor in stock for low quantity purchases.

    Mouser Electronics has the inductor in stock:
    https://www.mouser.com/ProductDetail/Murata-Electr...

    An alternate capacitor available from Mouser is:

    https://www.mouser.com/ProductDetail/Wurth-Electro...

    Were you able to source the other components?
    Did you fabricate your own PCB or purchase it from OSH Park?

    Regards
    Walt

    0
    barryc61
    barryc61

    Reply 8 months ago

    Hi Walt,

    Sorry for delay is responding. I ordered the PCBs via OSH and was delighted with the order. For 9 PCBs I paid ONLY $5.10 - in total including shipping to the UK!!
    Absolutely brilliant service! SMT components sourced as you kindly suggested from DigiKey.co.uk. Not arrived yet but looking forward to building the project.

    0
    dialup_prisoner
    dialup_prisoner

    8 months ago

    Nice work Waltx! The work you put into the PCB makes it possible for people to etch a single sided board at home, which is really useful! I was unimpressed by the commercial offerings too - as you say, they all seem to focus (pun intended!) on brightness at the expense of battery life. Your example should be much more useful for me, and probably many others.

    Is there any chance you could share a PDF of the PCB layout? Or your Gerber files? Kicad is supposed to be able to open Eagle's .brd files but it fails on this one.

    Otherwise if anyone else is interested, let me know here and I'll transcribe from Waltx's excellent schematic.

    Thanks again Waltx!

    0
    dialup_prisoner
    dialup_prisoner

    Reply 8 months ago

    Thanks again, that PCB displays perfectly on Kicad here. Can I ask how you did the export? It's a long time since I've used Eagle, and it certainly wasn't trivial to export to Kicad back then!

    The schematic doesn't work so well for me. No worries if you don't have time, but you may need the cache.lib file which I think has the schematic symbol information... but I'm happy to do the schematic myself too :-)

    Does Instructables accept .zip files? That would be a way to package up an entire project folder...

    0
    waltx
    waltx

    Reply 8 months ago

    I imported the eagle 7.7 project into KiCad (5.1.5)-1 using the "Import Project" under the file tab.
    Instructables would not except a .zip file.
    Here is the cache.lib file. Again, you will have to remove the .txt extension.

    0
    dialup_prisoner
    dialup_prisoner

    Reply 8 months ago

    Thanks again. I wish I could vote for this project twice!

    0
    waltx
    waltx

    Reply 8 months ago

    I will add the gerber files or KiCad files.

    0
    psargaco
    psargaco

    Question 8 months ago

    I'm working on the design of a specialized LED light to use with my old Kindle. The plan is to use 3 bright red LEDs and reuse one 18500 battery from a PS Move. I've tried connecting the LED in parallel each connected in series with its own resistor and it works fine, but after reading your 'ible it seems obvious that I'll need some sort of driver. I've found multiple options to buy one (please forgive me for not building it myself) but will I need one driver per LED?

    0
    waltx
    waltx

    Answer 8 months ago

    The circuit used for this project is a constant current boost regulator. As designed, it will also drive three LEDs, in series, with a constant current of 25mA.
    You have not specified the current required by the LEDs that you are using, so I cannot offer any guidance on the current.
    Several quick measurements are:
    battery voltage supply current LED current
    2.7V 76mA 24.9mA
    3V 59mA 24.9mA
    4V 50mA 24.9mA

    0
    psargaco
    psargaco

    Reply 8 months ago

    I see. However, assuming I wanted to connect 3 LEDs in series, I would need at least 6V supply, right? And I'm not sure I understand what is the "supply current" value you mention on your table? Attached you can see the LEDs I have. They're standard red 5mm LEDs.

    0
    waltx
    waltx

    Reply 8 months ago

    You will not need a 6 volt power supply. The circuit is a boost regulator. It will increase the voltage to the LEDs to a value that will force 25mA through the LEDs. The circuit regulates the current through the LEDs to be 25mA.
    5MM LEDs have a typical maximum operating current of 25mA. This circuit will be good option for efficiently driving your three red LEDs, in series, with one driver circuit. It will maximize your battery life.

    0
    psargaco
    psargaco

    Reply 8 months ago

    Aaaah! Good stuff. Thank you for your patience.

    0
    psargaco
    psargaco

    Reply 8 months ago

    I clicked "reply" too soon

    IMG_20210204_190804.jpgIMG_20210204_190823.jpg
    0
    psargaco
    psargaco

    Reply 8 months ago

    Great, thanks.