The senses: the five senses-vision, hearing, smell, taste, and touch-are universally recognized as the means by which humans and most animals perceive their universe. They do so through optical sensing (vision), acoustic sensing (hearing), chemical sensing (smell and taste), and mechanical (or tactile) sensing (touch). But humans, animals, and even lower-level organisms rely on many other sensors as well as on actuators. Most organisms can sense heat and estimate temperature, can sense pain, and can locate a sensation on and in the body. Any stimulus on the body can be precisely located. Touching of a single hair on the body of an animal is immediately located exactly through the kinesthetic sense. If an organ is affected, the brain knows exactly where it occurred. Some animals such as bats can echolocate using ultrasound, while others, including humans, make use of binaural hearing to locate sounds. Still others, such as sharks and fish (as well as rays and the platypus), sense variations in electric fields for location and hunting. Birds and some other animals can detect magnetic fields and use these for orientation and navigation. Organisms can sense pressure and have a mechanism for balance (the inner ear in humans). Pressure is one of the main mechanisms fish use to detect motion and prey in the water, and vibration sensing is critical to a spider's ability to hunt. Bees use polarized light to orient themselves, as do some species of fish. And these represent only a small selection of the sensing mechanisms used by organisms. Sensing of course is not limited to higher organisms. It exists in all organisms down to the cell level. Some of these can be observed directly such as some that are associated with plants including sensitivity to light, heat, and moisture. Plants have exquisite chemical sensing mechanisms that allow them to detect and often to protect themselves from pests or the effects of cold weather. Even lower on the dimensional scale, some microbes can detect electric and magnetic fields and use these to their advantage. The range of sensing mechanisms and the range of their sensitivities are truly vast. The eye of a hawk, the hearing of a fox, the olfactory sense of a hyena, or the capability of a shark to detect blood in the water has always fascinated us. But, what about the ability of a moth to detect pheromones released by another moth at large distances or the homing of a bat on a single insect without being able to see it? Organisms also have a variety of actuators to interact with their environment. In humans, the hand is an exquisite mechanical actuator capable of a surprising range of motion, but it is also a tactile sensor. The feet, as well as many muscles, allow interaction with the environment. But, here as well there are other mechanisms that can be used to affect actuation. A human can use its mouth to blow away dust or sooth a burn and can close and open its eyelids, a cat can unsheathe its claws, and a chameleon can move each eye independently and shoot its tongue to catch a fly. Other actuators allow for voice communication (vocal chords in humans) or the stunning of prey (ultrasound in dolphins, electrical shock in eels), direct mechanical impact used by some species of shrimp, and many other specialized functions. Some actuation, such as the movement of a sunflower to track the sun or the twisting and turning of an oat seed to insert itself into the soil are more subtle but nevertheless equally important. With respect to the sensory and actuation diversity in organisms, we are still very far behind and our mimicking of natural sensors and actuators is still in its infancy. It has taken the better part of 40 years to develop a working artificial heart, whereas seemingly simple organs, such as the esophagus, do not yet have an artificial implementation. Where are we in comparison with the nose of a dog?

A person consuming 2,000 kcal/day, assuming all calories are expended in 24 h, produces 2,000 x 1,000 x 4.184 = 8.368 x 10 6 J of energy. That translates to an average power of 8.368 x 10 6 / 2 4/3,600 = 96.85 W. That is an average power of about 100 W, with lower values during sleep and higher values while active. During exertion such as exercise, a person can produce in excess of 1.5 kW. But the real story is that the body both expends energy and requires the resulting heat to regulate the body temperature and supply its energy needs. The body requires a fairly narrow range of temperatures. The normal body temperature for most individuals is 37 °C, but it can fluctuate somewhat, with women having an average temperature about 0.5 °C lower than men. At temperatures above 38 °C, the body experiences fever, an elevated temperature due to failure of the body to regulate its temperature (typically, because of illness). Hyperthermia is an elevated body temperature that is not a fever but a reaction to external heat, drugs, or stimulants. Above 41.5 °C, the body enters into a state called hypetpyre3da, a dangerous state that can lead to serious side effects or death. Lower body temperature, hypothermia, is equally dangerous. Defined as a body core temperature below 35 °C, it typically occurs due to exposure to extreme cold or extended immersion in cold water, but can also be due to trauma. Heat regulation in the body is controlled by the hypothalamus in the brain and can be accomplished using a number of methods, including sweating, increasing or decreasing heart rate, shivering, and constriction of blood vessels.

Electric and magnetic fields are too important and too common to be neglected by nature in its grand design. Many animals and organisms have found ways to take advantage of these fundamental forces for sensing and actuation. The electric field in particular is used for both sensing and actuation. Almost all rays and sharks can sense electric fields produced by prey, as can some catfish, eels, and the platypus. Electric fields are sensed through use of special gelatinous pores that form electroreceptors called ampullae of Lorenzini. Sensing can be passive or active. Sharks and rays use passive sensing; prey is located by sensing weak electric fields produced by the muscles and nerves in the prey. Some animals, such as the electric fish, can generate electric fields for the purpose of active electrolocation of prey. The same basic sensory system is used by young sharks for protection by freezing in place when electrolocation fields are detected. But perhaps, the best known example of electrolocation is the platypus, which uses electroreceptors in its bill to hunt by night. Actuation is just as common and is used primarily to stun prey, and also for protection. The torpedo or electric ray (genus Torpedinidae) is one of some 70 species of rays that can produce electric charge and apply it in a manner similar to a battery. The charge is produced in a pair of electric organs made of plates connected to a nervous system that controls them. In rays, these biological batteries are connected in parallel to produce low -voltage, high -current sources. The range is between 8V and more than 200 V, with currents that can reach a few amperes. Another example is the electric eel (Electrophorus electricus). Since it lives in freshwater, which is less conductive than seawater, it has its plates in series to produce higher voltages (up to 600 V at perhaps 1 A, in short pulses).

The ear is a sensor and actuator in more than one way. Essentially a mechanochemical sensor, it includes a moving mechanism on the hearing side of the structure. But the ear also features a gyroscope, the inner ear, responsible for stability and sense of position. The ear itself is made of the outer and inner ear. The external ear is no more than a means of concentrating and guiding the sound toward the tympanic membrane (eardrum). In humans, the external ear is a relatively small, static feature, but in some animals it is both large and adjustable. The fenec fox, for example, has external ears that are larger than its head. At the bottom of the ear canal, the tympanic membrane moves in response to sound and, in the process, moves an assembly of three bones, the malleus (connected to the eardrum), the incus (an intermediate flexural bone), and the stapes. The latter, the smallest bone in the body, transmits the motion to the cochlea in the inner ear. The three bones not only transmit the sound but also amplify it through lever advantage afforded by their structure and dimensions. The cochlea is a spiral tube filled with a fluid. The stapes move like a piston, moving the fluid that in turn moves a series of hair-like structures lining the cochlea. These are the actual sensors that release a chemical onto the auditory nerve to affect hearing. The inner ear also contains three semicircular canals arranged at 900 to each other, with two roughly vertical and one horizontal. They have a similar structure to the cochlea, including a series of hair-like structures affected by the fluid in the canals based on the position of the body. These serve to maintain balance and provide information on the position and attitude of the body. The effect of motion on these structures can be immediately seen if the body rotates as, for example, on a merry-go-round. We temporarily loose the ability to keep our balance. The ear is a uniquely sensitive structure. It can sense pressures as low as 2 × 10^-5 Pa (or 10^-12 W/m²; i.e., on the order of one-billionth of the atmospheric pressure) and can function at levels 10¹³ times higher. That means the dynamic range is about 130 dB. The nominal frequency response is between 20 Hz and 20,000 Hz, although most humans have a much narrower range. But the ear is also very sensitive to pitch and can distinguish very small changes in pitch and frequency. A 1-Hz difference between two sounds is easily detectable. The hearing in humans is binaural and the brain uses that to detect the direction of sources of sound. Many animals use the mechanical motion of the outer ear to accomplish the same function but much better than we do. It should also be noted that many animals have much more sensitive hearing than humans, with ears that respond to higher frequencies and to a wider range of frequencies.

The modern world has an almost innate fear of nuclear radiation. It may be the heritage of Hiroshima and Nagasaki or it may be that we just fear the unknown, the invisible, and of course there are some very good reasons to be careful. Nuclear radiation can cause damage to cells and in high doses is known to cause cancer or even death. However, radiation comes in many shades and forms. All electromagnetic waves fall in the same general category of radiation, the difference being only in frequency (and with it in energy). If one were to imagine an instrument with a dial that can change the frequency from zero to infmity, then as the frequency would rise, it would first generate low-frequency fields, first in the audio range, then into ultrasonics, then above about 200 kHz, into what colloquially is called radio waves. Further up, the instrument will pass through very high frequency (VHF), ultra-high frequency (UHF), and then into the microwave region. Beyond that lies millimeter waves and then infrared (IR) radiation, followed by visible light and ultraviolet (UV), then into X-rays, α, β, and γ rays, and further up into cosmic rays. As the frequency increases, the energy associated with the waves increases, and the radiation effects become more pronounced. As is generally known, UV and X-rays are harmful radiation and are part of the cumulative effect of radiation in our lives and health. It is expected that people working with X-rays will naturally be exposed to more radiation than those who may only have a scan in a lifetime. Pilots and frequent fliers will necessarily be affected by cosmic rays as are astronauts in space. But beyond these, there is a background radiation level more or less constant over the globe. It is a low-level radiation caused by radioactive isotopes in rocks and soils of the order of 20-50 becquerel/minute (Bq/min) that can be detected with Geiger counters. This radiation is of no consequence to health, as it is too low to do any damage. The exposure level is, on an average, about 2.4 millisievert/year (mSv/yr). But there are locations and conditions in which the background radiation can be higher and of more concern. Granite rocks and hot springs tend to have higher radiation levels, and certain areas around the globe have naturally occurring high radiation levels as high as 250 mSv/yr or higher. On the other hand, sedimentary rocks and limestone have lower levels. Underground locations, including quarries, mines, or even basements, can have higher levels primarily from radon (a decomposition by-product of naturally occurring uranium and its isotopes), and radon can be found in the atmosphere as well as in water. However, beyond reasonable caution, it should be remembered that these are natural sources that have been there from time immemorial and will be with us for any imaginable future.

The purpose of this chapter is to discuss the general issues associated with interfacing and to outline the more general interfacing circuits the engineer is likely to be exposed to. However, no general discussion can prepare one for all eventualities and it should be recognized that there are both exceptions and extensions to the methods discussed here. For example, analog to digital conversion is a simple -if not inexpensive -method of digitizing a signal for the purpose of interfacing with a microprocessor. However, this approach may not be necessary, or may be too expensive, in some cases. A case in point: Suppose that a Hall element is used to sense the teeth on a rotating gear. The signal from the Hall element is an AC voltage (more or less sinusoidal) and only the peaks are necessary to sense the gears. In this case, a simple peak detector, followed perhaps by simple signal conditioning, may be adequate. An analog to digital converter (A/D or ADC) will not provide any additional benefit and is a much more complex and expensive solution. On the other hand, if a microprocessor is used and an A/D converter is available onboard, it may be acceptable to use it for this purpose in lieu of adding circuitry.

Least square polynomials or polynomial regression is a method of fitting a polynomial to a set of data. Passing a polynomial through a set of data means selection of the coefficients so as to minimize, in a global sense, the distance between the value of the function y(x) and the values at the points. This is done through the least squares method by first writing the "distance" function.

In the following paper, we explore a few issues associated with integer and fixed point computation on microprocessors. We do not discuss floating point computation since floating point operations are rarely resorted to in interfacing in the context of 8-bit microprocessors.