Magnetic sensors
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Magnetic sensors In addition to their use in reed relays, reed switches are widely used for electrical circuit control, particularly in the communications field. Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. Examples are door and window sensors in burglar alarm systems and tamper proofing methods (however they can be disabled by a strong, external magnetic field). Reed switches are used in modern laptops to put the laptop on sleep / hibernation mode when the lid is closed. Speed sensors on bicycle wheels and car gears use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key; cheaper switches are now used. Electric and electronic pedal keyboards used by pipe organ and Hammond organ players often use reed switches, where the glass enclosure of the contacts protects them from dirt, dust, and other particles. They may also be used to control diving equipment such as flashlights or camera, which must be sealed to keep pressurized water out. At one time brushless DC electric motors used reed switches to sense the rotor's position relative to the field poles.[citation needed] This allowed higher-powered switching transistors to act as a commutator, but without the contact problems, wear and electrical noise of a traditional DC commutator. The motor design could also be 'inverted', placing permanent magnets onto the rotor and switching the field through the external, fixed coils. This avoided the need for any rubbing contact to provide power to the rotor. Such motors were used in low-power long-service-life items such as computer cooling fans and disk drives. As cheap Hall effect sensors became available, they replaced the reed switches and gave longer service lifetimes. In recent times, solid state Hall effect sensors have replaced reed switches in most of the uses described here. |
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