Solar Powering Your Models

Hi folks, Doc here. Historically, model designers and builders have turned to electric motors when they wanted to automate their latest creations. Most of us are familar with the wide range of choices in this area, from small low torque, high RPM motors to strong, heavy-duty high torque motors. Traditionally, the energy source, electricity, has been either stored in the form of dry cell batteries (direct current - DC) or was in the form of alternating current (AC). In this feature, we'll look at another way to produce the electricity needed to operate those motors: solar energy. First, I'll briefly discuss the science and technology involved, and then I'll show you how to incorporate solar power into your own model designs!

THE SCIENCE OF SOLAR
Solar energy is often used to charge storage batteries, which then provide power in the traditional manner. However, solar energy can also be used to generate electricity directly. This is the method that we will use to power our models. But, before we get to that, how can sunlight (or light from an incandescent source) be used to produce electricity? The answer is chemistry. When light falls onto a semiconducting material like silicon, some of the electrons in the atoms of that material are knocked loose. When these "free" electrons move out of the semiconductor, the result is an electrical current. The key to this process is to package the semiconducting material in a form that can be easily and efficiently used. The most common such form in use today, and the one that we will make use of, is the photovoltaic cell, a.k.a. the solar cell.

A typical solar cell is made from two layers of silicon. In one layer, the silicon is treated, or "doped", with phosphorus, in the other layer boron is used. Differences in the chemical properties of these two elements produces an electrical field at the boundary between the two layers, making one negative and the other positive. When light falling on the cell knocks electrons off of the atoms in the silicon layers, they move from the positive layer to the negative layer, producing an electrical current. By attaching wire leads to each layer, this current can be used to power lights, motors, etc.

SOLARIZING YOUR MODELS
Only two components are needed to utilize solar energy for model power: a solar cell and a motor designed for use with such a cell. Solar cells are delicate by nature, so they are commonly encapsulated inside sturdy plastic panels to make them usable in practical applications. Such panels can contain one or more individual cells, and can be made in any size. Multiple panels can be combined to produce large arrays where a particular application requires it. For model building, small mini-panels are available (see photo above). Such panels, usually only a few inches on a side, typically range from 0.5 to 1.5 volts and generate anywhere from 100mA (milliamps) to 500mA of current under direct sunlight, with 400-500mA being common. Such panels are readily available for hobbyists.

In the past, model builders have only had to match the voltage of a motor to that of the intended power source: battery or AC. However, if you want to use a solar cell as your power source, you must consider both voltage and current when choosing a motor. In fact, current may actually be more critical, since many small battery operated motors are designed to operate at or above the maximum current output of most mini-panels. But, like mini-panels, mini-motors designed to work with them are also readily available from hobby suppliers and other vendors. In fact, several OS manufacturers have added such motors and panels to their accessory parts, and even offer special solar construction sets (the panel shown above is from Eitech and the motor at right is from the Ami-Lac Solar Derrick set. Although parts from any source will work, OS motors and panels are matched to work together, and OS motors typically come with integral mounting hardware and pulleys.

A SOLAR POWERED MODEL: THE AMI-LAC PUMPJACK
The video below depicts the featured model from the previously mentioned Ami-Lac Solar Derrick construction set. The model is a simple, yet functional design that lends itself perfectly to solar power. It incorporates a small 1-volt motor and a solar cell panel that produces about 400mA of current under full direct sunlight. It will also operate, although more slowly, under a strong incandescent light source that is placed close to the panel. This model can also be easily replicated with parts from almost any other system. For more information on this model, including detailed photos a a complete parts list, see my Model Gallery feature here.