Hi, I would appreciate if someone could give me some help in modifying my Xmas beanie hat led, so it is a simple hi/off switch and not a hi/med/lo/off switch, as I am have to prod my forehead several times to turn it off! I’m not great with circuits, have included the picture below.
I’ve read the other threads on mode modification, modified a cheap 3 mode torch with the pencil over the capacitor trick which worked, but doesn’t seem to have any effect on this circuit on either capacitor as it doesn’t have memory, just 3 modes then off. I’d appreciate if someone can point me in the right direction, I will update if there is success with pics for anyone else that may be looking for the same “fix”. Thanks
Can you read the part number or marking code on U1 and U2?
U2 looks like a microcontroller chip that is programmed by the vendor; that program would have to be changed, or a new chip installed and programmed by you for the switch control scheme that you desire.
Do B+/- go to the battery terminals? Where does the red and black wire run, V+ and V- ?
Hi thanks KennyBobby. U1 looks to be “LTH7” or I looked in another one I’ve now pictured that has a similar circuit that is “LTH7R”. Unfortunately the microcontroller U2 has no markings on either. B+ And B- runs direct to the 3.7v 250mah battery on the back of the board, pictured this too. V+ and v- run direct to the usb charging slot so will be a 5v input.
I didn’t realise it might be programming or soldering in another microcontroller. I was just looking to change it to a simple on/off setting. Happy to desolder the chip if possible and just join tracks if that can be done?
Can you read a part number on the U3, i missed seeing it earlier.
The U1 is a LTC4054ES5-4.2 from Linear in the first case, and from FUMAN [SHENZHEN FUMAN ELECTRONICS CO., LTD.] in the second. It is a charge management chip from the USB to the battery. It also appears to have some involvement from the U2 for the charging indicator leds.
It appears that they etched the part numbers off of U2 and several other items to attempt to hide the identity. That would slow down the process, but it could still be determined if it were really important—it is such a pain when they do this.
The push button sends a signal to the U2 controller chip which controls the transistors that actually switch the LED 1-4 to make the H,M,L levels.
Thanks again for your help and efforts, and explanation.
I think u3 beside the battery is “”3a9a” on the green board and the blue board is “3a01”.
I wondered I’d changing the momentary push switch to a latching switch and doing away with the U2 controller would work, if the only loss of other things it controls is the charging leds which wouldn’t bother me, unless it has more functions than that?
I’ve taken photos of the underside of the board, and flipped the images so they are corresponding to the top pictures of it helps showing the tracks to decipher what is going on? The blue board didn’t work out so well and isn’t clear to the naked eye but thought I’d add it anyway.
It looks like transistor Q1 is used to switch all 4 LEDs to ground and is driven from one of the pins of U2.
Can you detect if the light is being switched on/off at a high frequency? You might see it as a flickering when waving the light and taking a video with your phone camera. Somebody on here has a video of what it looks like.
This switching is called pulse width modulation (PWM) and is used to vary the average current and could be used to create the 3 different levels. A Current-sensing Resistor RC1 marked 3R3 having a value of 3.3 Ohms is used to measure the current and feed it back to the U2 controller.
It might be possible to bypass U2 and just use a small direct drive circuit.
How long will it run on Hi mode now? Does it have an automatic shut off if the battery voltage goes too low?
Hi, thanks again, I’ve Taken a video and a slo mo video of the light but can’t see any flickering, and read a little on pwm, what I have also done is tested the Voltage on a led whilst on, full mode is 2.85v, mid is 2.45 and low is 2.2v, I’m not sure if testing voltage over an led would show pwm though?
I will get a run time test on it later, as I usually use it in short bursts at night, but at a guesstimate it is probably 20-30 minutes. It usually just runs dimmer when getting low, I’m pretty sure there is no automatic shut off when voltage has dropped down, again I will check later.
I believe I have the same beanie light. Thanks to this thread I realize its rechargeable. Charging it now. Mine has PWM for sure. Just put on low and swing it fast, it will look like its blinking.
you might have to run the light on Low, then rapidly wave it in a diagonal while it is being filmed at normal or high speed (if that exist), then play it back on slow motion, in order to see the switching pulses. There may be other tricks for this and hopefully someone else will chime in.
The other way is to use an oscilloscope or some sort of fast data recorder such as a Saleae or maybe an Arduino device to capture the actual waveform
Another issue is that the pushbutton is only a momentary switch and you would need something that has a solid ON and OFF position.
That thing is just too smart for it’s own good when you want to make a change.
If you could mount a small slider switch for the on/off, then just need a resistor to limit current and some jumper wires. It could bypass the circuit as is, but still also allow for its use if desired.
Thanks was having a look for a small smd latching switch (4-6mm) I could solder on, also looking at flip flop modules that convert a latching switch to a push switch, smallest boards look to be 10x15mm but space is limited to mount them. Happy to use a slide switch too if it is small enough.
If you could let me know where I could bypass the circuit from and to that would be great, and any suggestions on what size of resistor I should add? Thanks again.
