soft latching FET based tail clicky switch

Flashlight Modding and DIY Parts
1

Has anyone ever ran across the idea of making a high current FET based soft latching power switch PCB as a replacement for the clickies switches?

I ran across this on adafruit


link to schematic

It is a 3A off/on soft latching power switch (we could definitely find a better higher current FET for sure) and design OSHPark boards for customization

Using a simple and cheap tactile button

Instead of having to find heavy duty built physical clickies with thick heavy contact wires use a simple FET to control the on/off function of the light

Imagine being able to have a 20A+ (or whatever the FET can handle) switch capability from a simple replacement board and using a quiet push button tactile switch

2

May help Push Button ON-OFF Soft Latch Circuits, Battery Powered Touch Toggle ON OFF Switch, Momentary Button MOSFET Power Switch for Microcontrollers
Fig. 3 is really simple and working good.
Not sure, but just change FET for hi-power and ready :wink:

3

nice…I saw that site as well…very cool!

Thanks

Same IC as mentioned in the schematic above, but rated at 7.3A
http://www.mouser.com/ProductDetail/International-Rectifier/IRF7389TRPBF/?qs=%2Fha2pyFadui6MTyr%2FeltINA9%252boO2CO27CWUAyVHn0mYUekZvpFc4zw%3D%3D

edit - wrong image…

4

Won’t you need a coin cell to get power to the switch?

5

not sure…the battery in the flashlight is the power source

The problem is, it looks as if the polarity is reversed

6

Looks like all that’s lacking is somebody to design a board find a MOSFET, solder it together and stuff it in a tailcap. Know anybody like that???
The quiet soft switch would be very nice, I’ve seen some tactile switches that do produce a click if desired.
Seems like the only current limitations would be the FET and the traces.

Could these also be designed to replace the momentary side switches with a remote mounted board?

7

I must be confused. That schematic looks to be on the plus end of the battery rather than the tail cap end.

8

didn’t tofty make a mosfet version of his 10a+ version tail-cap?

9

yeah, I just realized that…doh, care to assist in finding a schematic that might work?

10

Why cant you run the battery in backwards?

11

um…wouldn’t that run the driver backwards, (reverse polarity)

I think problem with the schematic is it needs power to be on before it can toggle…since the driver is “off” and no current flow from the + side of the battery terminal to the ground (the body) it won’t have the required voltage to activate the FET’s

Only way to have this work “independent” of the actual battery in the flashlight is like RBD said above, it will need it’s own power source (aka the button cell) but that kind of defeats the purpose

Arrrgh…the longer I look at it the more I understood why it wouldn’t work as is…dagnabit, unless a CR927 was somehow incorporated into the spacer needed for the PCB or something

12

arrgh…I see no way to run it as is…crapolio

13

wait…I just tested my Ultrafire F13 and my SH98 (SK98 clone from ft)…touching the negative lead to battery - , and the positive lead to the body (bare metal) I get battery voltage, the light doesn’t turn on but the battery input voltage IS there.

I think if the input is connected to the flashlight body and the ground to the negative spring it might work…investigating further

Check it out yourself…pull the end cap, check with volts on multimeter, put the red on the body/threads/bare metal of flashlight, the black on the battery - and you will see the battery voltage but since no current flow the light won’t turn on…I do believe this is doable

14

The switch will need some voltage. Since power goes through the driver and the LEDs first then you won’t have full cell voltage to operate the FET. Mattaus was working on one I believe but with a coin cell. Side switch lights could use one. Don’t think this is worthwhile for most one or two cell lights. Maybe a 3s cell light where one or more of the cells has a pos terminal at the tail end of a battery carrier or a carrier modded with a pos feed to the tail.

15

I think tofty is a beefed up physical copper clicky…all the metal contacts inside are much thicker and copper to handle more current

This type would be more or less a solid state switch, that can handle ALOT of current

16

hmmm

I guess others have been discussing it

17

That’s where I was going to point you. Abandon all hope ye who…etc.

18

You are right that there is a electrical potential when you use your dmm like that. Until you turn on the switch... Then there is very very little.

19

It must be brought positive to bring into the MOSFET conduction;

just use a lithium backup battery.

20

This is a good idea, but it would work best if the FET is placed between the battery and driver.
This way you can connect to the positive (button top of the cell) and negative (ground ring of the driver). That way a coin cell is not needed.
I guess that most flashlights that have a small clicky switch on the side use a scheme similar to this one.