Nuts & Volts - April 2005

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-cm²cell.

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

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Circle #98 on the Reader Service Card.

An Example — Solar Power With Current Monitoring

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