Elenco Green Projects - Manual (Page 25)

Project #21 Clock
Capacitor Project #22
0.5mA
0.5mA
Capacitors store electricity in an electric field between metal plates, with a
small separation between them. This electric field is similar to the magnetic
field of a magnet. Compared to batteries (which store energy as separated
chemicals), capacitors can only store small amounts of energy, but they
can release it quickly, can be made in very small sizes, and are inexpensive.
Build the circuit shown. Set the meter (M6) to the 0.5mA
setting. Flip the slide switch (S5) back and forth to charge
and discharge the 470
μF capacitor (C5).
With the switch in position C, a electricity briefly flows
from the battery into the capacitor to charge it up, as
shown by the meter. With the switch in position B, the
energy in the capacitor discharges through the red LED
(D1), which flashes.
The meter only measures current in one direction, but
you can flip it around to measure the discharge current.
Build the circuit shown. Set the meter (M6)
to the 0.5mA setting. The clock display will
light, but the meter will not measure any
current. See page 4 if you would like to set
the time.
The clock needs only about 0.005mA of
current to operate, and this is too small to
measure on your meter. The battery can
run the clock for a long time without being
recharged.
The clock uses a liquid crystal display (LCD) to
show the time. LCDs use very little power, but
cannot be viewed in darkness. The electronic
circuitry that keeps time, controls the display, and
allows you to set the current time is complex but
has been miniaturized in an integrated circuit (IC).
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See projects 1 & 3 if you need to recharge the battery (B4).