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.

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

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.

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.

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.

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

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.

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