cells in most small electronics applications, the efficiency is only about five
percent. The crystalline silicon cells
used for most domestic power systems
are typically about 10 percent efficient,
so we can only get about one watt out
of a 10 x 10-cm2 cell.
Note, however, that we will have
much less power available in dim,
indoor conditions. Our eyes are very
adaptable, so we can see over a wide
range of light levels (full moonlight
has a million times less light per unit
area than full sunlight), but a solar
cell’s short-circuit current (and therefore power at peak-power conditions)
will be proportional to the light intensity. Consider the floor of a six-meter-wide room lit by a 100-watt light bulb.
Even if the walls and ceiling were perfect mirrors, a robot on the floor is
only going to receive about three
watts per square meter. A 10 x 10-cm
cell and five percent conversion efficiency would only produce about 1.5
milliwatts of electricity.
Devices for indoor operation and
devices with very small solar cells
must, therefore, either have microwatt
power levels (calculators are one example) or must trickle-charge an energy
store and operate only periodically.
This is the “solar engine” circuit used in
many BEAM projects: A capacitor is
trickle-charged, essentially starting out
by charging with the high short-circuit
current from the cell, then trailing off as
the capacitor charges up. Eventually, it
will almost reach the open-circuit voltage of the cell, only drawing enough
current from the cell to balance any
leakage. However, a threshold switching circuit, using a flashing LED or a
138 voltage switch, will instead discharge the capacitor when the voltage
has reached a useable level.
has been proposed for exploring Mars
and one was recently tested in
Antarctica. I wanted to explore how a
lightweight, solar-powered version
could be made by attaching flexible
solar cells to the wall of a beachball,
using a Basic X- 24 microcontroller to
measure its motion with an accelerometer (see the Frisbee Black Box project
in the February 2004 Nuts & Volts).
Because a solar-power-only rover
could have its power interrupted by
shadows, I used a small battery for
short-term operation, which would
then be topped off by the solar panels.
Driving the microcontroller required 25
mA or so at six volts or more, so I used
a set of three tiny nickel metal hydrite
(NiMH) batteries, each with two cells.
These were so small that they could be
The Power to Design...
C I/ O
Development Tools & Kits
Kits for most micros, CPLDs, and more starting at $15…
• Universal CPU Board
• PIC and AVR Kits
• Programmable Logic Kits
• SeaBass Basic Compiler (as low as $10!)
• TTL RS232 Adapters
• PC I/O for Basic, C, Java, Linux, and More
Add powerful features to any microcontroller
project. Perfect for use with Basic Stamps!
Starting at under $10...
• Floating Point & A/D
• PWM & Pulse Output
• Servo Control
• Pulse Input
• PS/2 Keyboard or Mouse
C Programmable AVR Kit
Xilinx CPLD Kit
Starts at $49.95
PIC Programming Kit
Starts at $29.95
Visit us for free tools, tutorials and projects
Circle #98 on the Reader Service Card.
An Example — Solar
Power With Current
One project I have recently begun
is a Tumbleweed rover, a windblown
sphere that can traverse a long distance by rolling in the wind without
using motors to drive it. Such a vehicle
:$17(' '($/( 56 $1' ,03257( 56
)RU (8523(¶V 1R (GXFDWLRQDO (OHFWURQLF ., 76
6$0 ., 7
6HULDO , 2 FRQWUROOHU
)RU +RXVH $XWRPDWLRQ
&DQ DOVR EH XVHG WR FRQWURO 029, 7 2:,
5RERW $UP WUDLQHU E\ \RXU FRPSXWHU
6XSSOLHG ZLWK :LQGRZV VRIWZDUH
) 81 ., 76
::: $5(;; & 20
3OHDVH FRQWDFW XV E\ H PDLO LQIR#DUH[[ QO