I’ve been retrofitting 7135 chips into all kinds of bad lights to make them regulated. The newest victim is a keychain flashlight called the S11. I bough it from Aliexpress for $5.
After taking it apart, the main LED is driven by a 3R0 resistor. There is also an empty pad below it. The space makes perfect place for a 7135 regulator chip.
*** Be sure to disconnect the battery before proceeding!!! ***
Desoldered the 3R0 resistor and cleaned all 4 pads using copper braid.
Using electrical tape to block the negative connection to the LED.
Soldered on the L7135 regulator chip in the position of 2 resistors. The ground pin of the chip is connected via top tab to the ground pads. The left output pin is connected to LED negative. Connect a wire from LED Positive (also VCC) to 7135 VCC pin on the right. Measure all 3 pins to make sure there is no short. The middle and right pins are isolated by electric tape and should not be connected to anything on the PCB.
Reconnect the battery. Put everything back together and you have a keychain light that will provide ~130 lumens max and is fully regulated in all modes till battery is near empty. The modes on this light are implemented using slow PWM so 7135 works fine in this condition. Some users might be suspectable to the stroboscope effect so be aware of it. Max mode doesn’t use PWM and is not affected.
The battery is measured to be 250mAh so when run in full brightness (350mA from the L7135 regulator) it will provide about 40min runtime. After modification it is brighter than stock even with a full battery and is a lot brighter when the battery runs down. I also replaced one of the lights with a 519A 2700K and another with a Cree XP-G because the angry blue stock LED is bad and some fake SST-20. The mode spacing is still garbage (something like 25%, 50%, 75%, 100%), but at least low modes are usable now. Since before the mod when battery runs down the low modes become way to dim.
Also needs to mention that the original claim of 400 lumens is complete bullshit LOL. No way it can sustain that brightness. After the mod it is actually brighter, so I suspect with the 3R0 resistor driving the LED it only gives less than 100 lumens max.
CORRECTION: I’ve got the connection of the 7135 chip backward and it was not working as intended. After more messing around and I finally connected it correctly as below. Now it is running at 350mA tested at the battery connection.