Have you ever realized that your senses constantly warn you against possible dangers from your surroundings? As you cross the road, for example, if you hear the horn of a car approaching at high speed, you immediately glance in the direction of the sound and thus avoid an accident that might otherwise be fatal.
Some dangers, however, are beyond the scope of sight and hearing. In some situations, the sense of smell successfully performs a warning function. Of all the potential hazards in your home, you can detect a gas leak, for example, only through your sense of smell. The first sign of a fire beyond your field of vision is the smell of smoke. People with weak or non-existent senses of smell are defenseless in the face of such situations.
Certain electronic devices have been developed to warn against such dangers. In the designing them, the human sense of smell was taken as the model. For example, gas or fire detectors produced along these lines are just crude imitations of the nose.
As you know, fire detectors react to smoke particles in the air and emit a warning alarm. Consider the models that work according to the principle of ionization. (Figure 20) These devices contain a special detection compartment filled with ions—electrically charged particles. So long as clean air enters the device, these particles' electrical charge remains stable. In the event that smoke enters, however, the ions are neutralized and the flow of electrical current is reduced. The drop in the current sets off a buzzer or other alarm.
(Figure 20) The picture shows the comparatively complex structure of a smoke detector. The system in scent perception cells, however, is far more complex than this.
The special compartment in these electrical devices can be compared to the scent-receptor cells in the nose. You have already seen how the electrical charge in the receptor cell changes as a result of complex processes, and how a specific message thus emerges. The mechanism in a smoke detector is a rather primitive model of the perception system in the scent-receptor cells. Also, the difference between a fire detector and the human nose is far greater than that between a spacecraft and an oxcart.
The Electronic Nose
(Figure 21) Some sensors used in the electronic nose. These sensors were designed under inspiration from the receptors in the human nose, but are incomparably more primitive than the receptors in the human nose.
(Figure 22) The functioning system of the electronic nose
The human olfactory system can distinguish some 10,000 different odors. A professional in the perfumery business is able to smell a perfume that has a 100 different odorants in it, and list the ingredients. This superior creation in the human nose has encouraged numerous scientists to design similar apparatuses and various research and development centers worldwide are trying to reproduce the marvelous scent-perception system in human beings. One model developed along these lines is known as "the electronic nose."
Instead of the human nose's receptors, made of proteins, its electronic equivalents employ a series of chemical receptors. (Figure 21) Each of these receptors is designed to detect a specific scent; as their selective capabilities increase, production of the devices grows more difficult and their prices rise. The sensors gather signals from their surroundings, turn them into binary codes by means of electronic systems, and send them to a computer. Electronic systems may be compared to the nerve cells in the olfactory system, and the computer itself as an imitation of the human brain. The computer is programmed to analyze data transmitted to it, thanks to which it interprets the signals in binary code. (Figure 22)
Electronic noses developed in this way are used in various sectors, especially the food, perfume and chemical industries and medicine. Universities and international organizations provide major backing for such projects. Nonetheless, as stated by Julian W. Gardner of Warwick University, "We're at the early stages of the technology"