00:00Very simple 4.2 volt battery charging circuit. It can accurately charge and indicate the status.
00:06The circuitry is very simple. Take a NPN transistor, a 47 ohms resistor and Zener diode for constant voltage.
00:14A pair of red and green LED is used to indicate the charging status.
00:18This TL431 can control the LED light on and off.
00:23680 ohm resistance and 1000 ohm resistance.
00:28Finally, connect the input to the USB plug and output to battery socket.
00:33You can charge a single battery with this circuit.
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