Maglite 2AA Conversion to a White LED
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). 134
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. 635