Inspired by this cute LED night light on Amazon.com(http://amzla.com/t4catdgl3rqt),
I decided to make a light-control Christmas light with my old light string.
It's so simple without battery protection function, but I don't care because I just use an old 18650 battery which is cheap.
So firstly, I designed this simple circuit.
On the left part, it's the charging module. The solar pad transforms sun light to electricity, via a diode that prevents reverse flow, the 18650 battery would be charged.
On the right part, the light sensor controls the voltage of Vbe(voltage between B and E). When Vbe>0.7, CE switchs on, then the light turns on. So when will Vbe>0.7? At night, the resistance of light sensor increases and the voltage of Vbe increases as well until the transistor "switch on".
Then it's something about handcraft.
Some tools I used, the lectric iron is the most important one.
Just place the parts on the board according to your preference.
Finally, connect the string lights and solar pad and test it.
When the circumstance is bright, it's off.
When I cover the light sensor with my finger, it's on.