Velleman EDU02 - Manual (Page 17)

17 www.vellemanprojects.com
Required parts: Solar cell, BC557 transistor, 4K7 resistor (yellow, purple, red, gold),
yellow led, battery holder for 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 line. 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 line).
SOLAR CELL
BC557
4K7
or ange le
For advanced users:
The led turns off when the batteries are removed. Why ?
In the simple battery charge circuit, there was a diode to
prevent discharging of the batteries in low light condition.
In this circuit, it has been omitted. Why ?