Some time ago I bought a cheap reading light for my kindle and surprise surprise it was quite useless with one white 3 mm LED powered from one coin cell battery. So I modified it into a sort of Frankenstein's reading light, with LiPO battery, 1 W LED and brightness control.
- 1200 mAh LiPO battery;
- integrated LiPO charger;
- low battery voltage indicator;
- light brightness control;
- max 1.1 W power.
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Step 1: Schematic
Circuit consists of 3 main parts:
- boost converter based on MCP1661;
- one cell LiPO battery charger based on MCP73831;
- adjustable low input voltage indication circuit.
PCB was designed with KiCAD. You can access design files from my GitHub. In order PCB would be reusable for future projects, I tried to design it that all three parts of the circuit could be cut out of the board and used separately. Also I added several resistors connected in series rather than one component in some cases, that way values can be selected more flexibly for different circuit purposes.
Low voltage indicator turn on voltage can be adjusted with RV1 potentiometer. 1 cell LiPO battery is nearly discharged when its voltage drops bellow 3.6 V. You can set threshold by connecting 3.6 V to battery terminal and adjusting RV1 potentiometer until D2 indicator LED barely starts glowing.
MCP73831 charger IC has 2 variants, one with a tri-state logic output for status LED, other with open-drain output. So be aware of that if status LED doesn't work as expected.
Step 2: Assembly
- Cut off original LED and battery housing.
- Sandwich battery, flexible lamp stand and PCB with glue or double sided tape.