do touch-sensitive lamps work?


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 or . That's because the human body makes a pretty good antenna. There are even small 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 . 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.

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