Sensor switch

A touch switch is an electrical device for controlling lighting that is different from the usual presence of a sensor. In foreign practice, devices are rightly called electronic. And rightly so, the achievements of solid-state electronics are actively used in the composition of the touch switch.

Key features of

touch switches Touch switches are named for use as part of the sensor( Eng. - sensor).Able to register the heat of the hand, mark the touch, focus on the sound. Such devices are called presence sensors and the lighting control for them is secondary. More often, complex electronic devices take on the role of security guards or controllers of various processes. A typical example is the automatic doors of a supermarket.

In a sensor switch, the sensor is physically unable to generate a strong signal to control directly. The voltage( or current) level is in millivolts( milliamperes).This is not even enough to transmit a signal to the base of the transistor. The second feature of the touch switches is the presence of amplifying devices. Usually, these are transistors or other representatives of solid-state electronics, cascading: the first is highly sensitive, but low-power, then coarser, but capable of pulling a heavy load. Often galvanic isolation of circuits is used with the help of optocouplers, where the control signal is transmitted through optical radiation( light).This separates the fragile sensors from the power section of the touch switch.


switch In addition to the optical, a radio band is also used. Then the broadcast medium becomes the air using the wireless communication protocols WiFi, BlueTooth, etc. The structure includes active elements, and they need energy to power them. It turns out from batteries or straightening the mains voltage and trimming to the desired level. The simplest example would be a parallel-type stabilizer on a zener diode. And very rarely there is the possibility of embedding a full-fledged switching power supply.

Depending on the type of sensor, the lighting responds to different stimuli. For example, clap your hands, voice command, a wave of your hand, or SMS from a smartphone. This is not a complete list of services found in the Smart Home system. In the latter case, it becomes possible to truly intelligent management of electronic building stuffing. Otherwise, the light can be switched on by an irritant, and turned off, for example, by a timer. That is inconvenient and does not contribute to energy savings.

Varieties of touch switches

Touch switches are remote or local. In the latter case, they are located in close proximity to the switched lighting power circuit. In the volume of one topic there is no possibility to examine in detail all types of touch switches. It is useful for the reader to familiarize with the alarm systems known today. Many touch switches have borrowed the principle of action from the field of protection.

Passive Infrared Sensors

Now a lot of attention is paid to the subject of passive infrared sensors( PIR) in security systems. These sensors respond to heat emitted by the human body. To avoid false alarms, the width of the active spectrum is trimmed on both sides. The switch is triggered by the peak radiation of the body with a temperature of about 36 degrees Celsius. Usually, the sensory system consists of at least two receivers of optical radiation in order to determine the angular position of the object of irritation: a person enters the room or exits.

Lighting Sensors

In this case, the sensitive areas of photoresistors( phototransistors) are directed differently. Then the signal on them is different, judging by the difference of the angular position. This achieves a different goal: the device is designed to respond only to moving objects, thus minimizing the chance of false alarms. The person usually does not remain calm, alarming( alarm).It is easy to protect oneself from such systems by wearing an ordinary space suit. But in the lighting system, such tricks are not relevant for an obvious reason: the visitor, on the contrary, wants to be noticed. Thanks to the possibility of determining the direction, individual sensor devices operate in dimmer mode: you will wave in the first direction - the light becomes brighter, in the second one it is muffled( Leviton products).

The touch switch is set to trigger on the chosen kind of visitors. Suppose a person sitting in a wheelchair, a child will not be noticed if the sensor is suspended too high. Allowed to provide the room with explanatory inscriptions: wave your hand through the window. This is required if you do not want to spend electricity on pets. Despite the presence of wool, all living things differ in temperature from the environment.

Infrared sensors are not able to cover the entire room physically. For the trivial reason, the most effective touch switches based on them are pass-through. Placed at the beginning and end of the corridor or stairs. When applying the delay off, it becomes possible to use in storage rooms, utility rooms. The truly useful sensor systems on passive infrared radiation are paired with an intelligent controller that will count people who have entered and left the room. Of course, any clever of hooliganism motives such a tandem will try to deceive, it is reasonable to supplement the touch switch and controller with auxiliary means.

Using the touch switch

Piezo elements

Piezo elements in touch switches are of two types, the principle of which is laid down, respectively:

  • Piezoresistive effect - changing the resistance of the sample under the action of mechanical loads.
  • Piezoelectric effect - the formation of potential differences on the faces of a crystal under the action of mechanical deformation.

Both effects were discovered in the 19th century. The chronology coincides with the order in the list.

Piezoresistive Sensor Switches

Piezoresistive Effect( term introduced in 1935 by John V. Cookson from the University of Wisconsin, from Greek piezo - crush) described by Lord Kelvin( Journal of the Royal Society, volume 8, pages 550-555, 1856-1857), note from June 17, 1857 about the study of the conductance of commercial wires for the telegraph) on the example of iron, platinum and copper. Perhaps a statement on the topic of increasing sample resistance within 0.5% in response to strong and numerous bends along the entire length is only a stretch of relevance to the topic. But historians disagree. Lord Thomson investigated the reasons for the differences in conductivity of samples used in the navy and derived a simple formula: the supplier of copper is important. Deformations affect the resistance to a small extent, it is permissible to neglect.

Thomson was aware of the effect of mechanical tension. And at the presentation of the Royal Society Prize( Baker Lecture, 1856), he reported on a curious experiment. On the shoulders of the measuring Whitson bridge, it included conductors of copper or iron of the same length, but some samples were stretched by suspensions. The device on the diagonal recorded the difference. Thomson explained this by mechanical deformations. But it is not known for certain whether the emergence of the experiment is coherent with the investigations carried out with respect to the telegraph wires. On top of this, readers can familiarize themselves with the order of numbers of resistance changes( abscissa axis) in the figure taken from IEEE's Proceedings for 2009.

Measurement procedure

Then numerous works of a similar nature followed. In the 19th century, these were notes by Tomlinson, and in the 20th century, Bridgman and Rolnik. The first interesting results were obtained in 1932 by Allen, who established the anisotropy of changes in crystals of zinc, cadmium, antimony, bismuth and tin. As for other studies, Bridgman's ideas led to the creation of tensor equations describing the process. In 1938, thanks to the efforts of many scientists, the first sensors were born. Like the ones that are used today in floor scales and transform the deformation into a change in resistance. Already in 1950, Bardin and Shockley predicted a significant piezoresistive effect in regular crystal structures three years before the discovery.

In its current form, the piezoresistive effect was born on December 30, 1953, thanks to the engineer with the common surname Smith from Bell Laboratories, who described the curious behavior of silicon crystals and germanium of both types of conductivity. Due to mechanical effects, the samples changed resistance. The Master of University of Connecticut Western Reservation was actively interested in the anisotropic properties of semiconductors and the work of Bardeen and Shockley. New sensors appeared already in 1950 with a sensitivity 50 times higher than analogs of pure metals.

Kulite Semiconductor, founded in 1958, became the first company engaged in the production of piezoresistive sensors. In modern models, buttons are created on the basis of a thin semiconductor membrane. When you press the center at the edges there is a strong tension that changes the conductivity of the site. Measurement is carried out by the bridge circuit or other methods. The unbalance voltage is amplified and serves to control the on and off of the light.

Piezoelectric Sensor Switches

The piezoelectric effect was discovered in 1880 by the brothers Jacques and Pierre Curie. As in the previous case, the phenomenon was predicted in advance. Based on the theoretical background, René-Just Gauy and Antoine César Becquerel suggested a possible connection between electricity and mechanical deformations. The first successful experiments were made on quartz crystals, tourmaline, topaz, sugar cane and segnevite salt. Yes, many substances exhibit piezoelectric properties:

  1. Human bones and tendons.
  2. DNA molecules.
  3. Dentin and tooth enamel.

A year later, Gabriel Jonas Lippmann suggested, based on the basics of thermodynamics, the existence of the opposite effect: deformation of crystals under the action of an electric field. This conjecture was confirmed in 1882 by Jacques and Pierre Curie, along the way they created a piezoelectrometer, used to study radioactive elements. In 1910, a textbook on the physics of crystals was published by Voldemar Voigt.

The effect caused a close attention of scientists. In 1917, a sonar for submarines( Paul Langevin) appeared on the background of the First World War, and in 1921 the first quartz resonator( Walter Gayton Cady) appeared. The development of the search led to the discovery of barium titanate in 1946( Arthur von Hippel).In the postwar period, quite a lot of applications of the piezoelectric effect appeared, but all of them were little connected with the topic under consideration. As for the control devices, we note two of them, in both cases using polymer films as sensitive elements:

  1. US3935485 on the piezoelectric keyboard. The purpose of the device is not specified, but looking at the names of the applicants( Kureha Kagaku, Kogyo Kabushiki, Kaisha) and year( 1976), suppose that the assembly was intended to control automated assembly lines on conveyors.
  2. Declared in US4343975( 1980), a sample can be seen by anyone even today on electronic scales in a store. This is a backlit keyboard, which makes the work of the operator much easier.
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