Project 5: Solar Battery Charger With 'charge'-lndicator
A led turns on when the batteries are charging...
Required parts: Solar œil, BC557 transistor, 4K7 resistor (yellow, purple, red, gold), yellowled, battery holderfor two AAA batteries, two AAA 1.2V rechargeable batteries.
How it works: When the sun shines, a current flows from the (+) of the solar cell via the Emitter/ Base of the transistor trough the batteries and back to the solar cell. This is the Base current, indicated with the dotted Une. In our example, the Base current will also charge our batteries. The fact that there is a current flowing between Emitter and Base causes the transistor to turn on and fully conduct, as if it were a switch. Hence, a current can flow from the solar cell via the transistor Emitter/Collector and resistor to the led and back to the solar cell. This current causes the led to light (solid Une).