Velleman EDU02 - Manual (Page 23)

23 www.vellemanprojects.com
Required parts: Solar cell, BC557 transistor, 4K7 resistor (yellow, purple, red, gold), 470
ohm resistor (yellow, purple, brown, gold), BAT85 diode, yellow led, battery holder for
two AAA batteries, two AAA 1.2V rechargeable batteries, jumper wire.
How it works: When the sun shines, the
voltage generated by the solar cell will be
higher than the voltage of the batteries, so a
current will flow from the solar cell to the
batteries. This current will charge the
batteries. The BAT85 diode prevents
discharging of the batteries trough the solar
cell in low light conditions. The base of the
transistor is tied to ground (-) by means of
the 4K7 resistor. This causes the transistor
to turn on and allows a current to flow from
the batteries trough the transistor, the led
and via 470 ohm resistor back to the
batteries. The led will turn on. However,
note that the base of the transistor is also
tied to the (+) of the solar cell, so as long as
the sun shines, the base of the transistor is
kept high enough to prevent turn-on of the
transistor, so the led remains off at daytime.
SOLAR CELL
BC557
orange led
R1
4K7
D1
BAT85
R2
470
C
har
ge c
ur
rent
Led current
(only at night)
B
as
e c
ur
rent (
onl
y at ni
ght)