This mod combines aspects of two of my recent mods, the e-switch in the tailcap mod and the sliding resistor switch mod. It uses a voltage divider with an adjustable resistor to continuously control the gate voltage to a MOSFET.
In the tailcap, the MOSFET needs a separate power source to turn on. In my tail e-switch mod I used a Li ion coin cell. I would prefer to use a supercapacitor instead of a coin cell, but the attiny13A MCU requires 2mA while on, which is too much for a supercapacitor of this size range (~0.3F). With the present mod I am not using an MCU, so I can control the current draw better which allows for the use of a supercapacitor. The current draws are from the voltage divider and from the charging bleeder resistor (while on). So making these resistances as large as possible minimizes the current draw.
I chose a variable resistor of 2 Mega ohms. The other resistors in the divider are 1 Mohm on the (+) side and 470 Kohms on the ground side. The charging resistor in parallel with the LED is 100 ohms and the one in the driver is 100 Kohms. While the light is on the MOSFET is basically open and the capacitor is draining at (3.8V/100Kohm)=38 microAmps. This is the main draw that limits the duration the light can be on. With a 0.33 Farad capacitor (link is external) this means the voltage is dropping by (38microA/0.33F)=0.4V/hour, so the light can only remain on for 1-3 hours, depending on the charge level of the capacitor. I could use a 5Mohm variable resistor to improve the on-time some. The capacitor is kept charged by the main cell to a voltage ~0.2V below the main cell. See my above two threads for factors affecting resistor choice.
I made a “pill” for the tailcap. From the top: the retaining ring with MOSFET soldered, a PCB with spring, spacer, supercap and resistors, 20mm PCB with variable resistor attached.
Attaching the MOSFET with a good thermal path was easier in this case than the last mod. The MOSFET is simply soldered to the retaining ring. Since the MOSFET is now in the tailcap the FET drain doesn’t have to be electrically isolated from the flashlight body like in my last mod where the FET was in the head.
I glued a 15mm PCB to the resistor dial and added three nubs for thumb grip. The white plastic spacer acts as a bearing so I can press on the dial without stressing the resistor itself.
That particular resistor is not robustly built and it stopped working before long. It is actually rated for only 200 cycles. The next resistor I tried was this one (link is external). It is rated for many more cycles and hopefully will last a long time. This resistor is a bit taller than the last one which prompted me to alter the thumbwheel design.
That is a 22mm PCB. Now the ergonomics are quite good. I can quickly and precisely change the brightness one-handed in the overhand grip. The dial is the only switch, so all the way counter-clockwise for off. I put a little “dead” space at the beginning so it’s unlikely to be accidentally turned on.
At the front I have triple dedomed XPG2 S4 2Bs under the carclo 10507 optic. Draws 9-9.5A on a fresh 30Q and does ~2300lumens at start and ~26Kcd.
Thanks for looking!