Electronics > Projects, Designs, and Technical Stuff
Cheap Chinese Cree XM-L T6 modification
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0xdeadbeef:
Well, call me eccentric but I really dig cheap Chinese flashlights. Most of them are the Cree Q5 type which take a single AA NiMH battery as they have some kind of boost converter built in,
but recently I bought some slightly larger ones called Cree XM-L T6 that need a single 18650 cell.

Anyway, while all of my Q5 type flashlights are single mode (on/off), the T6 flashlights are usually multi-mode.
Which by itself isn't necessarily a bad thing, but nearly all of them have an annoying strobe mode.
Now I got my hand on two Ultrafire XM-L T6 flashlights with three modes: high, low, strobe.
The bad thing is that the things remember the last mode for a real long time. So if you used high mode, put the flashlight aside and switch it on a few minutes later, it will switch on in low mode.
Even worse: if it was in low mode before, you turn it on in strobe mode. And that's really the most annoying strobe ever.
Actually the type likely to trigger an epileptic seizure.

So I took the thing apart and found surprisingly few parts inside. The main part is a P-FET, namely a Fairchild  FDS9435A with the marking P51QJ and a mystery SOT-23 with the marking HA4E.
Typical highside switch configuration where the FET connects the load to the battery voltage.
The gate of the FET is connected to the SOT-23 and there's a 3k resistor between gate and source (VGS=0 -> off) which again is connected to the battery's positive pole.
Last but not least, the resistors drain is connected to the anode of the LED (L+) through four parallel 1Ohm resistors.
The LED's cathode (L-) is directly connected to the common ground which is connected to the minus pole through the case and the mechanical switch.

So it's quite clear that the mode handling and PWM/strobe creation is done by the SOT-23. I measured the gate in all three modes to see if the high mode used a PWM - which it didn't:
   high:   100% DC
   low:    75% DC, 230Hz
    strobe:    50% DC, 7.7Hz
   
So one solution would be to bypass the SOT-23 and connect the gate directly to ground.
However I assumed that storing the mode was only possible through the capacitor (~1.6µF) and tried to remove it.
Unfortunately, this didn't really improve the situation much. So I replaced it by a 22k resistor. This
effectively converted the flashlight to a single mode one. Yet it felt wrong to remove the capacitor completely
and I wondered if adding a resistor in parallel would still allow to use all three modes but reset the stored mode faster,
so the flashlight would always switch on in high mode after a few seconds off.

So I put the capacitor back and botched a 22k resistor in parallel on top. With this modification, the mode is always
reset to "high" after about 2-3 seconds off while before it "remembered" the last mode for minutes or even hours.
The two other modes are still available if you switch the lamp off/on faster than the 2-3seconds needed for resetting the mode.
I guess I might try 10k resistor the next time to reduce the off phase needed even a bit more.
Vgkid:
Thanks for this post, I always contemplated getting one of those XML-t6 lights, but never did.
0xdeadbeef:
They are pretty cool and quite a bit brighter than the Q5 type with a much broader beam. Still I think the Q5 type is the best compromise between size, brightness and usability. These are also much cheaper even though they have more complex electronics (boost converter). Besides. the 18650 cells frighten me a little bit - even though they are available with protection. For the moment, I use LiFePO4 cells but they have lower voltage and capacity compare to Li-Ion.

Anyway, be aware that there are quite a few different types of XM-L T6 flashlights available. Most have 5 modes (low, mid, high, strobe, SOS) and while I assume they can be modified with a similar approach, you can never be sure what kind of surprise another flashlight model will unveil. Also my version didn't have the dents in the LED module, so it was a bit tricky to unscrew it.

Side note: I found it surprising to see that the P-FET (FDS9435A ) used in my flashlights is not really specified for voltages below 4.5V. The RDS will be quite a bit higher for typical Li-Ion voltages and even worse for LiFePO4.
I tested the module with a bench power supply at 3.7V and the current increased from like 350mA to slightly over 500mA. Still, while the P-FET got a bit warm (around 45°C or so), it didn't look critical and the main heat source is the LED anyway.
nukie:
They use whatever they could source cheap, ie fish of the day

You will also notice some pcb designed to accept multiple footprints.
0xdeadbeef:
Hey, that's part of the adventure  ;D
Actually, I ordered dozens of Q5 flashlights over a longer period - most of them as presents for relatives, colleagues etc.
And every time I got slightly different ones: different switches, with/without dents to unscrew the LED module, with/without silver foil as reflector, even the size and other mechanical details differ.
It seems there was a base model that all the clones are based on, but probably in the meantime everybody forgot what the original was ;)

Anyway, I really wonder who came up with that multi-mode idea. Two or three brightness levels can make some sense but I find it hard to imagine when someone would need a fixed strobe mode or even a SOS signal on his flashlight.
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