The device is connected with a USB connector to the mobile phone charging adapter.
The transistor has a gain of beta=200.
A potentiometer 25 kiloohms simulates a photoresistor. In the dark, its resistance is high, the base of the transistor is fed through a 12 kiloohms resistor, and the transistor opens.
When the photoresistor is illuminated by, for example, daylight, its resistance drops substantially, connecting the base of the transistor to the negative terminal of the supply and closing the transistor.
