A strain gage is at the heart of many electronic sensing systems. A strain gage is a thin, flexible wafer, measuring about one half inch by one inch. It has two terminals, with output wires attached to the terminals. When a strain gage bends, its resistance changes. Strain gages are glued onto different surfaces. As a surface bends, the resistance of the gage changes. This change in resistance is measured by electronic equipment. In addition, a computer system is often used to present data to engineers. An example of this is in experimental aircraft. The strain gage is glued to the wing. As the airplane flies, engineers can measure the amount of flex in the wing by reading the strain gage's change in resistance.
A load cell uses a strain gage to measure pressure. A load cell is a solid block of metal with a strain gage glued to it. Load cells are commonly used in scales, both large and small. For example, large scales weigh trucks at most truck stops. Load cells are placed beneath the platforms. When a truck drives on top of a platform, the load cell bends just a little bit because of the strain. The strain gage's resistance changes, and the computer can display an output reading of the weight, based upon how much the load cell's strain gage bends. All electronic scales are very accurate, measuring down to the ounce or even to one one-hundredth of an ounce.
The human nose is a very sensitive organ. However, a dog's nose is even more sensitive. Engineers seek to mimic the characteristics of noses via the use of electronic sensors. Researchers at the University of Texas claim it is possible to use chemicals with properties that change electrically. For example, a drop of a chemical can put out or transmit a small electrical charge when carbon monoxide touches it. Conversely, the resistance of another chemical may change when exposed to propane. These electronic "noses" have very useful applications. Gas detectors in mines are possible. Natural gas found in deep underground mines puts out a scent, undetectable by human or even dog noses. If even a few molecules of natural gas touch the sensor's chemicals, an electrical signal is transmitted. A computer interprets the signal and alerts the miners that they have hit a natural gas pocket.
Some smoke detectors use a light and a small photovoltaic cell sensor to trigger an alarm. This happens when a small light shines on a cell, leaving a small gap between the light and the cell. The cell's voltage output is dependent upon the amount of light that shines on it. As the light level dims, so does the output. If smoke fills the gap between the light and the cell, it blocks the light, so less light shines on the cell. Internal circuitry reads the drop in voltage transmitted by the photovoltaic cell and triggers an alarm. It is a very simple system, yet it can save lives in the event of a fire.
A thermocouple, which is a type of electronic thermometer, is very simple. In 1821, researcher Thomas Seebeck discovered that if two different metals are placed together, a voltage is produced when the assembly is heated. This is called the Seebeck effect, and thermocouples use this principle. For example, two wires made of different materials are wound together. When the assembly is heated, a voltage is produced. The greater the heat, the higher the voltage. The voltage output is then fed to electronic circuitry, which displays a temperature based upon how much voltage is produced.