Designing into flashlight housing

This is my first try at designing electronics to fit into a flashlight case. I have some questions, which may seem very basic.

I have selected the Convoy L2 host based on size, battery configurations, and general look and feel, and have samples of the host in hand.

I see mentions of the tailswitch controlling modes of operation. I assume this is an interaction between the switch and the driver electronics. Is this done by sensing a momentary switch opening, or is something else going on? I see holes for a spanner wrench to remove the tailswitch from the tailcap. Are there other switches with interesting options I can populate here?


There is a space above the batteries, below where the LED would go. I assume I need to create a PCB for that position to hold the negative battery spring and pass that wire out through the holes in the bulkhead. Is there anything else "special" about that space?


I will be doing the mechanical work tonight, creating an OpenSCAD shape that matches the inside of the light housing, which I will be using when designing the electronics onto the two (?) PCBs which will sit in that space. I'm sure there will be at least two boards, and that they will stack with some number of wires running between them. That part is easy. Fortunately the shape of the backside of the reflector is already a pretty close fit, so I will start with that and modify till I have a shape that just fills the space. I'm trying to pack a lot of electronics in here.

DVH8 said:

Are there other switches with interesting options I can populate here?

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One feature some people enjoy are illuminated tailswitches. Usually a simple circuit that drains a small amount of the current for illumination. The SMD LED would glow throw a transluscent rubber cover or annular ring around the cover. Helpful for locating the light in the dark.

In earlier times people might resort to using the tailswitch to effect a dimming or multi-mode feature, but these days that's handled with the "driver" (what you refer to as the PCB electronics).

DVH8 said:

Is there anything else "special" about that space?

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Things to bear in mind about this area of the light include heat management and circuitry mounting/ ruggedness. The LED itself and some of the components on the Driver generate a fair amount of heat that is best reduced as mush as possible for electronic stability, protection of component life and efficiency reasons. A good design will permit heat to move quickly away from the area. Some designs use external fins, some use copper or other means of additional mass. Depending on the level of ruggedness, some people encapsulate the cavity to protect the electronic from shock (and heat). Finally there are some common dimensions in this area (e.g., 20mm opening with a 0.5mm ledge to rest the PCB) that people design around so that off-the-shelf driver boards can be used.

I don't have any special knowledge, just what I read. Happy trails partner.

That light and almost all lights with tail switches don't really sense the button. The button interrupts the power to the driver and the driver has clever means of measuring how long the power was off for.

Not knowing your background, please excuse me if any of the following is just repeating things you already know.

DVH8 said:

This is my first try at designing electronics to fit into a flashlight case. I have some questions, which may seem very basic.

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Where you may want to start is simply by customizing an L2 yourself with available components, then take the big step up to designing your own electronic components, something I don't think many us step up to. This will give you a decent idea what is already available and how it works, so you can think about changes you might like to make to bring something new

The Convoys are great for this because there is a good selection of available components. The L2 takes a 20mm PCB for the driver that controls the light, and a 20mm PCB (almost always metal cored / MCPCB and usually with a direct thermal path for the LED for heat dissipation to the body).

The driver can be as simple (although not very versatile or efficient) as an appropriately sized and wattage rated resistor, or microcontroller-based multi-mode design. You can get some drivers and other components from Convoy's official store on Aliexpress, as well as other options from places like Mountain Electronics. There's also some specific driver designs created by flashlight community members sold through other channels, like the LED4Power LD-series drivers (I think they have their own site), and the Texas Avenger series (details on that one are posted on Budget Light Forums by users Texas Ace and Lexel).

There also are some community-developed firmwares already available for ATTiny-based drivers, suited to a couple common power topologies (eg - powered only by 7135 regulators vs. a combination of 7135 regulated low modes and FET driven high modes).

I see mentions of the tailswitch controlling modes of operation. I assume this is an interaction between the switch and the driver electronics. Is this done by sensing a momentary switch opening, or is something else going on?

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Momentary button controlled designs act as you suggested, but not mechanical switch controlled designs like the L2. As Parametrek said, it is usually based on figuring out what happened before or while the power to the driver was interrupted by the rear switch. Some select their behavior for the next power cycle while the light is powered on, based on how long it has been on (on time mode control). Some select their behavior when powered on based on how long it was off (off time mode control). The simplest way to implement off-time control, which arguably is more intuitive for most people, is with a small capacitor connected to one of the microcontroller input pins, so the firmware can decide what to do based on the remaining voltage from the capacitor when the power comes back on.

Most commonly among drivers designed for enthusiast modding, these functions are controlled by an Atmel ATTiny series microcontroller. This includes for the standard Convoy driver, as well as many aftermarket drivers like Qlite, H17F, and I think Texas Avenger. The microcontrollers are then paired with linear regulators (type 7135 usually) or FET's to control the power to the LED. The LED4Power designs are a bit more complex, I believe.

There is a space above the batteries, below where the LED would go. I assume I need to create a PCB for that position to hold the negative battery spring and pass that wire out through the holes in the bulkhead. Is there anything else "special" about that space?

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Right. That space is there for the driver. I've only seen single PCB driver designs, but I'm sure some lights, probably including the L2, have room to fit two boards in there. Here's an example of a driver based on an Atmel processor, 7135 powered low modes, and FET powered high modes, that also supports multiple different firmware choices:
http://www.mtnelectronics.com/index.php?route=product/product&path=67_117&product_id=714

Hope that helps and isn't simply rudimentary to you.