GFS16 - Battery Indicator Tailcap Light & 1mR FET Tailswitch! (Rev B)

Small stuff indeed. Amazing work. :beer:

Great! I have been waiting for years for this! A guy on CPF used to offer these, but with forward clickies and completely over-the-top prices.

Hello all, 1st I want to thank loneoceans for sharing this with us. just finished building 2 switches and they work with the H17F driver only if a bleeder resistor is added, I used 2.2Kohms and all functions flawlessly. My question to loneoceans or anyone who may know... would a lower value bleeder expedite the coin battery charging time.. with the 2.2Kohms it took about 3HRs off time to charge from 2.55v to 2.67v on a full 18650 battery, Thanks.

![img]https://i.imgur.com/64TojtG.jpg[/img]!

![img]https://i.imgur.com/bSa23Fr.jpg[/img]!

![img]https://i.imgur.com/eugZNBC.jpg[/img]!

![img]https://i.imgur.com/Q1uZNvW.jpg[/img]!

![img]https://i.imgur.com/N5EzwVN.jpg?1[/img]!

2.2Kohms bleeder is only good for the switch board alone, for both the switch and the led board... 3x 2.2Kohms or 733ohms worked perfectly.

Hello Fady, great job on the assembly of the boards, looks like you did a fantastic job and many thanks for sharing the images.

Fady, the small coin batteries are designed to be charged fairly slowly, and as you probably have seen during assembly, charging is from a small 3.3V regulator and a series 2.2k resistor, and is designed to charge the cell slowly. Now when the switch is on, the battery bleeds via a 1M resistor on the FET gate, so any losses would be through internal discharge, and parasitic loss through the fet and reverse flow on the regulator, which should be very slow.

The first time you're charging the coin cell from the factory, it should take a while. But once it's fully charged, the voltage shouldn't drop substantially any more after in everyday, normal use. Also I suppose compatibility of the GFS system will depend on which driver you're using.

Hi loneoceans, Thanks for your comments, I built nine so far and they working great except for 1 died within 12Hrsā€¦ The light was left on a tableā€¦ next day I noticed the red leds were on, I checked the 18650 voltage and it was fullā€¦ the switch wouldnā€™t work, after a bit of T/Sing found out that the coin cell went down to 0v, not sure if somehow it shorted out or was just a bad battery, I drew out the schematics of the switch circuit & made sure I do not have a short somewhereā€¦ so I went ahead and replaced the battery, all good now. I also used green leds instead of blue on some of them, P/N 150060GS75000.
A tip for anyone planning on building 1 is to superglue the led board on top of the switch otherwise the rotation force may break the coin cell solder joints, that happened to me.

Now that I have the schematics i and looked up the LDO reg. datasheet, what happens in the circuit once the coin cell voltage is equal to the REG. voltage? and whats the function of R2 also what do you mean by reverse flow on the reg, isnā€™t the diode there on the positive rail to prevent this? sorry loneoceans I know Iā€™m asking too many questions!! : )

For battery longevity, it's important to charge the battery at or lower than the manufacturer rated charge current. This is outlined in the datasheet here (https://www.sii.co.jp/en/me/files/2016/02/English_Micro-Battery-2016.pdf). Once the battery voltage reaches regulator voltage, the battery simply stops charging. Diodes are not ideal devices and there will always be a small amount of leakage current, likewise with the FET gate, and internal leakage within the battery. R2 is not strictly necessary but it reduces the initial turn-on gate drive current which will lead to a more consistent turn on performance (esp. since the micro battery has a fairly large internal resistance).

Note that the LED board and FET board are standalone so there's no requirement to use both. Also if you use the stacked-fet-switch stackup, with a little bit of filing, it's possible to also swap the battery out (MS412) to the bigger MS518.

Cool! thanks for the explanation, yes Iā€™m aware that both boards are standalone, I already have 3 of them built with the MS518SE battery, it fits best when using Q1 on 0.8mm board and the 1.6mm led board.

I really like the idea and the realization. Is it possibble to use 3x2 leds in 3 different colors for battery indicator? So that way maybe the switch can be green from 4,2V to 3,7V and orange from 3,7V to 3V and red under 3V?

With the current PCB design with no modifications, nope, but otherwise, yes very possible with different logic :).

Just asked because mostly I donā€™t discharge my batteries all way down and I will be happy with an earlier reminder.

If you read the original post, it's very easy to change the resistor divider values to suit your threshold voltages, and to also take into account any effect the series driver may have on the tailcap circuit. :)

edited with new info

Hi loneoceans, Iā€™m planning on changing some values to the followingā€¦ā€¦.

Vbatt x R4 / R3 + R4 =Vref

  • When battery level drops below 3.30V, Light up Red.ā€”ā€”ā€”ā€”ā€”ā€”ā€”ā€”- 3.3V x 360K/116K+360K = 2.5V

Vbatt x R6 / R5 + R6 =Vref

  • When battery level drops below ~3.00V, Turn off all LEDs.ā€”ā€”ā€”ā€”ā€” 3.0V x 360K/ 72K+360K =2.5V

Does this look right or should I decrease R4 to keep R3 at a lower value, sorry I looked up the datasheet and i couldnā€™t figure it out, whatā€™s the rule when setting R2 value in a voltage divider circuit pertaining to the MAX9052BEUA+ datasheet? Also how do I factor in the voltage drop across the bleeder resistor to have compensated values for R3&R5ā€¦.

Vi= Vt x Ri/ RB+R4+R3

So to set a trigger point of 3.3V, bleeder resistance= 100R, R4=360K, R3(compensated for bleeder)=115K. drop across R4= 2.5V

If any one need help setting different trigger point or led brightness, just PM me.

note a 100ohms bleeder is requiredā€¦ any higher value messes up the ref voltage for the compactor even if your calculations are correct, the video loneoceans posted implies that heā€™s hooking up a bench power supply without any resistance and Iā€™m sure his driver must have some sort of resistance from positive to ground other wise itā€™ll short circuit, also not shown is an internal hysteresis of about 0.02V where both channels are trigger.

Also note that the 100R bleeder will be in parallel once the flashlight is turned onā€¦ consuming 37mah average, a minimum of 1/5 watt power rating required, 1/2 watt recommend for reliability to cope with the heat generated from the led, easily available in 0603 package.

I would add a 2.8V LDO to the tail cap, so you get constant brightness of the LEDs

R4/R6 should be large to have a low current over the voltage divider, so do not lower them
their current draw is even present when all LEDs are shut down

The reason the calculated brightness varies from calculated resistors id that the bleeder consumes voltage,
the drop differs with the current drawn from the LEDs, so constant current is at least easier to calculate

for example if your 6 LEDs draw together 0.3mA then on a regular 100 Ohms bleeder the voltage drops 30mV
Each LED draws 50uA
LEDs are powered over a 2.8V LDO so we do not have to mess with different currents on different battery voltages
LV 3.2V
SD 2.9V

on this example the current drawn of the 3 regular LEDs is removed when you hit 3.2V
so the current drops from 0.3 to 0.15mA also the drop on the bleeder jumps from 30mV to 15mV
so the comparator notices when 3 LEDs shut down a voltage rise of 15mV
The question is how big is the hysteresis of the comparator is this basically dictates how big the bleeder can be

With such a low hysteresis not really big LED currents or big bleeder of 1kOhm are possible without flickering

I looked in the datasheet of the comparator and adding 2 resistors per channel from Output adds hysteresis,
this would mean a bigger bleeder possible

I also noticed the comparator shows 130uA supply current which is more than many low brightness tail caps consume, so storing the light without checking tail color is not a good idea

Using an LDO would pretty much remove the need of a comparator with internal reference

Thanks Lexel, makes a lot of senseā€¦ I finally got it to work exactly as I want it, changed LEDs current & R3/R5 - R4/R6 values works good with no hysteresis noted, ill upload a video in a little bit. I have the compactor set for 3.7V LV and 3.5V SD.

old values showing hysteresis

new values, works better

Video shows off the switch design nicely. Good job

credit goes to loneoceans, its his design, hope he shows up again with his driver release!

I have made a new enhanced version

2x3 colors above first shut down
1 single LED for low battery signal

28uA 2.8V LDO up to 5.5V
dual pad for a 10V LDO for 2S lights
capacitors for LDO

2.8uA comparator without reference voltage, significantly reduced standby drain
the reference voltage comes from the 2.8V regulator and a voltage divider

added for each comparator a hysteresis voltage divider, so a larger bleeder like 470 or 1000 Ohms should be possible

17/20mm version

20mm with trimmers for LED brightness

Wowā€¦ awesome job Lexel! Did you just design this in the past 24 HR :+1: !? Well when is this gonna be available to buy?