A concept for rear-mounted momentary switches (e-switch) without diameter increase
Background
In my opinion, a momentary switch (e-switch) is a must-have for flashlight user interfaces, as opposed to power cycle or “clicky” switches. Locating the switch on the tailcap is also much preferred over the side of the light. The combination of these two – a tailcap-mounted momentary switch – is the optimal setup for me, but this is difficult to achieve in normal cylinder-style flashlights. This is because the driver (at the front) needs to be connected to battery power at all times and have an additional signal from the switch (at the back) connected to it, with the battery blocking the way. Further complicating the issue is that most lights need the battery tube to unscrew from the head, so any connections must be able to rotate without issue.
A few manufacturers have offerings that do this: HDS/Ra, Novatac, & Klarus all have lights that use an additional element (spring, wire, or sleeve) to carry the signal from the tail-mounted switch up to the driver. One particularly innovative solution was briefly offered by Nitecore called “Piston Drive,” which introduced the concept of transferring the switch signal from back to front via mechanical means instead of electrical. All of these systems work well (in my experience), but they have a common drawback that they require an increase in the outside diameter of the battery tube to carry the additional element.
Concept
Inspired by the innovation of Piston Drive (that is, carrying the switch signal mechanically rather than electrically), I created this concept of Cell-Slide Switching to achieve a tailcap momentary switch without outside diameter increase. The core concept is simply to use the battery itself to translate the motion of the switch from the back to the front of the flashlight.
Implementation
So far I have successfully implemented Cell-Slide Switching on two lights – a Solarstorm SP01 and a Xeno G42. Both of these are old lights that I had in my junk bin – lights that I like the design of but never use because I can’t stand using clicky switch interfaces. Using CSS I have updated the user interfaces on them to MELDsc (single channel) which is a MELD UI implementation that removes all the color channels.
Modifying a light to CSS is fairly straightforward. The driver needs a spring on the back side of the driver to contact the positive end of the battery, some drivers need this to be added. A fairly large spring is used, so that a momentary switch can be mounted inside it. Mounting and connecting the switch can be a little bit complicated because the two switch connections need to be electrically isolated from the positive connection to the battery. I have tried two varying methods for this, shown below. I think a purpose-made PCB could make this very easy to do as well.
Once the switch is mounted the connections need to be made to the microcontroller of choice. In my case I am using the PIC16F1575 on a small breakout PCB, running MELDsc firmware. For one of these lights I replaced the driver electronics with AMC7135s; for the other I took control of the existing driver via a PWM signal.
The tailcap of the light needs to be modified as well. The new setup needs to have a spring that keeps the negative end of the battery connected to the tube during small translational motion of the cell. This turned out to be simple to implement in both lights I tried. I removed the PCB with the clicky switch, an d replaced it with a large diameter spring to connect the cell to the tube. The large opening in the spring allows for a small 3d-printed cylinder to sit inside of it. This cylinder connects the switch boot to the back of the battery so that pushing on the tailcap moves the battery forward. It took a few iterations to adjust the length of the cylinder to work well.
Disadvantages
Cell-Slide Switching has a significant disadvantage that the spring forces need a lot of tweaking for it to work well (the front spring needs to be stiffer than the rear; not too stiff or the switch’s tactile feedback is drowned out; not too weak or impacts will allow the battery momentum to click the switch). The length of the battery also is critical for the proper switch operation. For this reason, I don’t imagine that this concept could ever be used on a commercial product since the use of different batteries would require adjustments or new plastic cylinders.
Conclusion
For my own purposes, I am able to put in the time to make the tweaks necessary to overcome the disadvantages and make it work well. The best part about this concept is that I am able to implement it on many lights that I previously wouldn’t use or buy because I dislike clicky interfaces, but now all of the normal tailcap-switch lights are opened up to using a MELD user interface. I invite others to try this concept for themselves, and I would also offer chips with MELDsc for use with this if anyone is interested in that user interface.
Implementation pictures
G42: small pcb prepared for switch mounting:
wires pass through small hole in stock pcb:
switch mounted with superglue:
all parts wired:
click test with intended 18650 cell:
printed cylinder:
located inside tailcap spring:
SP01: switch mounted:
driver modified with MELDsc chip:
finished spring/switch arrangement:
test before modifying tailcap:
G42 with Cell-Slide Switching completed: