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FBTA
Fake Bike Theft Alarm
This
compact project shows how to safely charge a 3.7V Li-ion battery via USB
and blink a LED efficiently with ultra-low power consumption.
The idea of this project is to install a flashing LED inside the end of a bicycle handlebar. This may scare a thief and prevent the bike from being stolen. You only need to make a hole in the grip for the LED. Typically a handlebar inner diameter is 19mm, The PCB needs 18m. 
Blinking LED in a bicycle handlebar I
am using a
800mAh/3.7V Li-ion
battery,
size 14500 (AA).
The estimated operating time is more than 6 months after the first charging cycles.
There
seems to be available also a 1500mAh LI-ion battery model. You
can easily
recharge
the battery via the USB-C connector. The red status LED is indicating charging
and the green LED is lit when the battery is fully charged.
The blinking LED circuit is a Schmitt trigger oscillator which drives a LED by means of a MOSFET. 
The charger is using a Microchip Li-ion CC/CV (Constant Current/Constant Voltage) charger IC. It provides preconditioning, constant current and finally safe constant 4.2V voltage charging until the current is low enough, which after it stops charging. The IC provides also status LED drive for charge and full charged condition. See more in the schematics. Current consumption optimizing In the first revision (Rev A), the oscillator used NC7S14, which consumed over 1 mA due to its input staying near ½ Vdd. Revision B replaces it with Toshiba’s TC4S584F, a slower, 4000-series–like logic IC with wide voltage range (3–18 V). This change reduced oscillator current to just 20 µA. After this modification, the total average current is about 85 µA, including the LED's 1.3% ON-time. Downloads - Schematics Rev B - Schematics Rev A for comparison - BOM B - Layout - Gerber files Update 2025-04-20 OH7SV Back to other projects |