Tuesday, January 5, 2010
How do touch-sensitive lift buttons work and what do they respond to?
Switches that are sensitive to human touch 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.
- Capacitance - Touch-sensitive lamps almost always use this fourth property of the human body. The "capacitance" of an object is the capacity the object has to hold electrons. For example: 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.
source: http://science.howstuffworks.com/question42.htmSo, in certain situations, like if you're wearing gloves, the buttons might not respond as they're supposed to. If the switches depend on temperature, the gloves would not let your body heat pass through and so the switch will not react. If they depend on resistance or radio reception, the gloves would block the conduction. And if they depend on capacitance, the gloves would also not allow the electrons to pass through, therefore not giving the switches anything to respond to.So if you ever come across these buttons in a lift, use your bare hands. They work better.
your name @ 10:59 PM | your comment link
2010.Why is ice sometimes 'sticky'?Our bodies secrete tiny amounts of sweat, which is a salty fluid, onto your skin surface that actually makes your skin stickier. This is why we have it: for grip. If you then touch ice with your bare skin, the extremely cold surface of the ice causes the sweat to freeze on your finger. Because the sweat has got into all the nooks and crannies of your finger and freezes solid, it will form a very tight bond between your finger and the ice. You get stuck to the surface. If it’s an ice cube – it’s okay because there’s enough heat flowing through your fingers (usually to melt that transient freezing) then you can detach yourself. In the case of a -70 freezer or in the Antarctic, you have to be very careful about this kind of thing. If it doesn’t warm up, you can end up permanently frozen to the surface or injured quite badly. That’s why ice is 'sticky'. You get literally frozen to the spot.source: http://www.thenakedscientists.com/HTML/content/questions/question/2265/
your name @ 10:14 PM | your comment link
