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by game masta on September 10th, 2006

game masta

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How does a touch lamp work?

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Answers. 4 helpful answers below.

  • by libberz on September 23rd, 2006

    libberz

    A touch lamp detects your touch by looking for changes in the electric properties of the lamp's surfaces. It monitors these properties by putting a fluctuating electric charge on them. As electric current flows toward the bulb through the lamp's wires, it passes through an electronic device that places a high frequency (about 60 kHz) alternating current onto those wires. This added current causes the lamp's surfaces to take on a small fluctuating electric charge--first positive, then negative, then positive, over and over again. This surface charging involves electrostatic forces, which extend long distances between charged objects, and occurs even though the lamp's surfaces aren't directly connected to the lamp's wires. The more surface the lamp has, the more easily it can hold that electric charge--the greater the lamp's electric capacitance.

    When you plug the lamp in, the electronic device uses its fluctuating charge to determine how easy it is to add or subtract charge from the lamp's surfaces. In other words, it measures the lamp's capacitance. It then begins to look for changes in that capacitance. When you touch the lamp, or even come close to its surfaces, your body effectively adds to the lamp's surface and its capacitance increases significantly. The electronic device detects this increase in capacitance and switches the lamp's state from on to off or from off to on. The fact that you don't have to touch the lamp to affect its capacitance means that a touch lamp can have insulating paint on its metal surfaces yet still detect your touch. You can also buy touch lamp modules that plug into the wall and turn the lamp that's connected to them into a touch lamp. These modules are so sensitive to capacitance changes in the lamp that you can trigger them just by touching the lamp cord.

    quoted from http://rabi.phys.virginia.edu/HTW/incandescent_light_bulbs.html

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  • by davoomac on September 21st, 2006

    davoomac

    Nearly all touch lamps work because of a property of the human body called capacitance, which is the ability to store electrical charge. Most touch lamps employ an oscillator, which is a type of tuned-circuit amplifier that produces an AC current, that is, one that regularly switches its polarity back and forth. The oscillator is connected to the metal parts of the lamp. When you touch the lamp's metal housing, you introduce your capacitance into the circuit. Now the oscillator has to pump charge into a much larger surface area--yours plus the lamp's. That causes the oscillator to detune, or change frequency. The lamp detects this change and toggles the light on or off. Kitty has capacitance too, but since she's much smaller, hers is a lot less than yours and isn't enough to trigger the lamp. A larger animal like an average-sized dog would, in case any touch-lamp owner is looking to teach his pet a new trick.

    --SDSTAFF Q.E.D.
    Straight Dope Science Advisory Board

    http://www.straightdope.com/mailbag/mtouchlamp.html

    For convenience I included the Howstuffworks article below.. very interesting stuff! read on:

    Switches that are sensitive to human touch -- as opposed to switches that must be flipped or pushed to make and break a mechanical connection -- have been around for many years. They certainly have advantages, and the most important is the fact that dirt and moisture cannot get into the switch to gum it up or damage it. Over the years, many different properties of the human body have been used to flip touch-sensitive switches:

    * Temperature - The human body is generally warmer than the surrounding air. Many elevators therefore use buttons that are sensitive to the warmth of the human finger. These buttons, of course, don't work if you have cold hands. The motion-sensitive lamps you see on people's patios also sense the heat of the human body.
    * Resistance - The human body, being made mostly of water, conducts electricity fairly well. By placing two contacts very close together, your finger can close the circuit when you touch it.
    * Radio reception - You may have noticed that, when you touch an antenna, the reception gets better on a TV or radio. That's because the human body makes a pretty good antenna. There are even small LCD TVs that have a conductive neck strap so that the user acts as the antenna! Some touch-sensitive switch designs simply look for a change in radio-wave reception that occurs when the switch is touched.

    Touch-sensitive lamps almost always use a fourth property of the human body -- its capacitance. The word "capacitance" has as its root the word "capacity" -- capacitance is the capacity an object has to hold electrons. The lamp, when standing by itself on a table, has a certain capacitance. This means that if a circuit tried to charge the lamp with electrons, it would take a certain number to "fill it." When you touch the lamp, your body adds to its capacity. It takes more electrons to fill you and the lamp, and the circuit detects that difference. It is even possible to buy little plug-in boxes that can turn any lamp into a touch-sensitive lamp. They work on the same principle.

    Many touch-sensitive lamps have three brightness settings even though they do not use three-way bulbs. The circuit is changing the brightness of the lamp by changing the "duty cycle" of the power reaching the bulb. A bulb with a normal light switch gets "full power." Imagine, however, that you were you were to rapidly turn the power to the bulb on and off (say 100 times per second) -- then the bulb would only burn half as brightly because its duty cycle is 50 percent (half on, half off). "Rapidly switching the bulb on and off" is the basic idea used to change the brightness of the lamp -- the circuit uses zero percent (off), 33 percent, 66 percent and 100 percent duty cycles to control the lamp's brightness.

    http://home.howstuffworks.com/question42.htm

    Here are some other sites worth checking out on the topic:
    http://www.straightdope.com/mailbag/mtouchlamp.html
    http://www.madsci.org/posts/archives/may98/893276774.Eg.r.html
    http://www.newton.dep.anl.gov/askasci/phy99/phy99375.htm

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  • by BFE4Life on September 11th, 2006

    BFE4Life

    Well, provided that there is a working bulb and the unit it plugged in - you touch it, and it turns on.

    Sorry, couldn't help myself :o)

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  • by nfntjy on September 21st, 2006

    nfntjy

    Here is a "How Stuff Works" article on how touch lamps work:
    http://home.howstuffworks.com/question42.htm

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