A Tumbleweed rover made from a beachball and
thin-film solar cells (top and bottom) with a circuit
board carrying diodes, etc. in the center. Another small
board with a microcontroller is at right. By distributing
the components around the ball, it rolls more evenly.
distributed around the perimeter of the beachball. Pin 24
(Vin) of the BX- 24 was connected to the batteries (Vout) via
a switch. You might get away with a smaller battery, wiring
four cells to get 4. 8 volts to the Vss supply on the BX- 24, but
this would be much less tolerant of voltage drops as the battery discharged. Also, if you don’t need the speed, memory, or A/D converters of the BX- 24, you could use a BASIC
Stamp 2 or, of course, a PIC microcontroller.
I wanted to monitor the battery voltage using the BX- 24
as a data logger, but its analog-to-digital converter inputs
need zero to five volts. Because of this, the two 100K resistors are set up as a potential divider to take the seven or eight
volts of the battery down to this range (Vout/2 Monitor).
Now, I wanted to also monitor the current coming
from each of the solar arrays, since the power from each
would come up and fall off as the Tumbleweed rolled
around. Each of the arrays was a series string of four TX
3-25 thin-film modules, giving an open-circuit voltage of
about 15 volts total, but a peak-power operating voltage of
more like 10 volts.
D1 and D2 are blocking diodes to stop the battery from
100k
50k
I2 Monitor
R1
100k
R2
I1 Monitor
50k
D1
D2
Vout Supply
100k
6x1.2V
Vout/2 Monitor
100k
0V
2 x 4 x TX3-25
Circuit for the Tumbleweed power supply. This circuit
charges a battery when it can, and provides for monitoring
the battery voltage and charging currents. You can remove
the bits of the circuit you don’t need or clone the solar
array and charging monitor parts if you have more arrays.
If you have more than one array or even only one array
and a battery, though, you need a blocking diode for each.
discharging backward through the solar panels, which would
not only waste battery energy, but also could damage the
cells. An ordinary silicon diode will do, but it can be wasteful
(especially in low voltage designs), since there is a drop of
some 0.7 volts or so across it. I prefer to use a germanium
diode, since the voltage drop here is only 0.3 volts or so.
If the cells are not illuminated (or only weakly so), the
diode does not conduct and the current monitor output (I1,
I2 monitor — this time divided by three to get into the zero-to five-volts A/D range) will be low. If, on the other hand, it
is strongly illuminated, the voltage will be enough to cause
a current to flow into the battery via the diode and the relevant resistor R1 or R2. This resistor allows the current to be
sensed — a value of 100 ohms is about right. Imagine the
array is putting out 20 mA, then the voltage here must be
equal to the battery voltage plus the 0.3-volt diode drop and
plus R1 times the current, or 0.025 x 100 = 2.5 volts. From
the simultaneous measurement of the battery voltage and
this monitor, the charging current can be figured out. If you
don’t need to sense the charging current, you don’t need a
resistor here, and the circuit will be more efficient without it.
SOURCES
When to Use Solar Cells
F
o
r
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
SOLARBOTICS ( www.solarbotics.com) has a range of
amorphous Sunceram cells for small robot projects such as solar
engines.
EDMUND SCIENTIFIC ( www.scientificsonline.com) has
a range of solar cells, multi-cell panels, low-current motors, and
various experiment kits.
The Iowa Thin Film cells are available from JAMECO
( www.jameco.com) and are from three to 12 volts with a range
of currents and plastic coatings.
76
The most obvious applications for solar power are outdoor devices with modest power needs of a fraction of a
watt. Very low power applications and indoor systems can
be driven much more easily with an alkaline battery
(although note that a solar cell circuit might be lighter). For
powers of a few watts or more, solar power can be rather
expensive. On the other hand, solar driven devices can be
aesthetically pleasing and, for lawn lights or weather stations or other applications where it is convenient not to have
to replace batteries, solar power can be a bright idea. NV
APRIL 2005
