Data Sheet

-32-
39. History of Hand Crank Generators
Build the circuit, then turn the hand crank generator (94
& 99) and you will light up the heart LED (69) initially. If you
turn the hand crank generator (94 & 99) faster, you will
see the lamp (76) light and get brighter the faster you turn.
Did you know that hand crank generators date back at least to
the 1960s? Military units used special hand generators that
could fold up into a backpack. With a soldier turning the crank,
the generator could be used to power electronic equipment
while in the eld, far from any available power source.
40. Kirchhoff’s Second Law
Replace the lamp (76) with the bi-directional LED (71) and
replace the heart LED (69) with the star LED (70). lf you
turn the hand crank generator (94 & 99), you will light
up both LEDs at the same time. However, you might have
noticed that you have to turn the hand crank (94) faster
to make the bi-directional LED (71) light up as bright as it
did in project 24. Kirchhoff’s second law states: The sum
of the voltages around a closed network is zero. If a drop
in voltage is considered as a negative voltage and a rise in
voltage a positive voltage, then the following equation is a
mathematical representation of Kirchhoff’s second law:
V
star
+ V
bdled
+ V
hcg
= 0
This shows that the voltage drop across the hand crank
generator (94 & 99) must equal the voltage drop across
the star LED (70) plus the voltage drop across the bi-
directional LED (71). Since the voltage being generated by
the hand crank generator (94 & 99) is split across the star
LED (70) and bi-directional LED (71), you need to turn the
hand-crank (94) faster than in project 24 to get the same
voltage across the bi-directional LED (71).
1st level