Remote switch tail board with FET, so no added resistance

:beer: :beer: :beer:

I wanted to build this for a long time (but lack of electronic knowledge stopped me doing that).

I have probably misled myself, and you, about the PFET implementation, It isn’t really any different from the N version, i.e. still needs gate drive, which isn’t available at the tail, once the FET is turned on.

Sorry about that. I was mis-remembering a design for a gun torch, where the “driver”, actually just a FET, was in the head, and the remote switch hardwired into it at the head, not the tail. A PFET was used to switch the positive, it was a modified incandescent torch being used, not LED, and this seemed the easier way at the time.

I found the old CPF article where a similar method was used to replace a mechanical clickie tail switch, which were not so robust at high current in those days.

http://www.candlepowerforums.com/vb/showthread.php?296127-The-quot-FETtie-quot-500W-Tailcap-Switch-for-Z41-FM26XXX

The replaceable coin cell seems the best way, but if wanting something permanently soldered in, even potted, perhaps e.g. a Panasonic VL series, recharged through a bleed resistor and some simple method to maintain the correct 3.4V.

https://industrial.panasonic.com/ww/products/batteries/primary-batteries/lithium-batteries/coin-type-rechargeable-lithium-batteries-vl-series/VL1220

https://industrial.panasonic.com/cdbs/www-data/pdf/AAA4000/AAA4000COL17.pdf

Another application for this switch is to allow the use of longer cells in some clicks lights or as a tool in many homebrew builds. I’ll be waiting for the final parts list as I’ve too been waiting for something like this:) it’s been talked about a number of times but nothing ever really came of it. Looks like you’re making it happen. Yay!

I’m running a boss head with 2x IMR 16340’s so probably well over 2,000 lumens. Using a stock SF tape switch and have had no issues. I would certainly be interested in looking at this for a different build I’m working on.

I’m still fairly new to BLF, hope a cut-down image of a rifle forend is allowed

TexasAce and WarhawkAWG have shooting guns in their avatar, and they are still here :smiley:

I’m interested!

Can you put me down for a kit please?

Is there a list of remote switches for various lights around?

I thought I saw a remote switch for the Brinyte B158 but can’t locate it on GearBest right now.

I have ordered some switches that can be fitted in tailcaps, so you can convert the normal one to remote

Wonder how much resistance it would lower if u make a battery free (the coin battery) remote FET switch.

Like the remote switch is connected to the flashlight AND the FET.

Doesn’t current like to flow where there is least resistance?

Like when you press the switch the current flows thru the remote switch to the flashlight but at the same time activate the FET in the tailcap so the current flows thru there to the flashlight and this are lowering the resistance?

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The FET IS the switch for the flashlight, in this case. The ‘remote’ switch activating the FET is so that the total current of the flashlight doesn’t have to travel through the tiny wires of the ‘remote’ switch. It’s like having a relay for the ignition switch in your automobile so that the full starting current for the car doesn’t have to flow through the tiny wires of the ignition switch.

you have to use a coin battery to activate the MOSFET,
you may choose a rechargeable one or super capacitor with a bleeder on driver, but a 25 cent battery will last for over 100 main battery cycles at high level output and 10 years

If you want a coin cell free tail FET variant you need an additional wire from the driver to the tail

the external remote switch has only the coin cell current flowing through it

It is possible to mod the tail cap and board to have a conventional switch and the external switch

Yeah. I don’t really know how I was thinking, but since I’m not a electrician the idea was great in my head.

My idea was to use the cells in the flashlight to power the FET since the remote switch is powering the flashlight also and it could steal some of the current to activate the FET.
And if the FET gets broken the flashlight would still shine but with the extra resistance from the remote switch.
Like the FET is connected in parallel before the remote switch.

Thinking about its still feels like a good idea but since I’m not an electrician I calculate my idea to be 99% wrong since I don’t know how those FETs work.

also another member said, you could use a P-channel FET without battery

the problem is you can switch the light on, but then you have not a positive potential to switch it off again

the resistance of the FET is 4-5mOhms at 2.4-3V, so probably less than a conventional switch, I see no problem add a normal switch in the tail and connect it to the 2 pads for remote switch and use the light that way
you will need 3 mm additional length in the tail cap assembly to make it fit

I received the FETS and battery holders today, but still waiting on the boards

Once you're bothering to do this, what about just placing an mcu there and doing PWM from the tail FET and be done with it all? One less fet, even less resistance. Less space for 7135s though but really fet+1 is fine.

Of course if someone wanted to just design a light from scratch just for this, placing a small hole down the length of the battery tube and passing a wire through it for an e-switch connection from the tail with a standard driver in the head would work but that's getting a bit tangential to the already very nice improvement.

This doesn’t activate the FET? Maybe need some resistors and maybe a diode too.

Ps deleting this post after I got my answer.

Interesting… Lexel will say :slight_smile:

Of course I see now that there's not really any room for an mcu or a 7135, and by the time you do run that wire, might as well do it all in the normal e-switch way. Short of that, this is nice.

A N-channel needs positive to be switched
a P-Channel negative

in your example you can switch the FET on but not off
when the FET is conductive no matter what you do the gate sees Ground and will be make it switch through
to make the P-FET close you need a positive voltage on the gate, that requires positive voltage in the tail

This is a no-go too?

you cant beat physics if the FET is conductive you got only ground potential available in the tail cap
only a 2. voltage source in the tail cap can give the P-FET a signal to close

to have no parasitic drain issue and a FET with very low gate Vgs a n-FET is the way to go

Going to read that a few times to try learn.

Just to make sure here, FETs are normally closed when not activated?