Simple battery-powered alarm clocks make a loud buzzing or beeping sound to wake a sleeper, while novelty alarm clocks can speak, laugh, sing, or play sounds from nature. Digital ĭigital alarm clocks can make other noises. In an electronically operated bell-style alarm clock, the bell is rung by an electromagnetic circuit and armature to turn the circuit on and off repeatedly. In some models, the metal cover at back of the clock itself also functions as the bell. Traditional mechanical alarm clocks have one or two bells that ring by means of a mainspring that powers a gear to quickly move a hammer back and forth between the two bells or between the interior sides of a single bell. Most modern televisions, computers, mobile phones and digital watches have alarm functions that turn on or sound alerts at a specific time. A progressive alarm clock can have different alarms for different times (see next-generation alarms) and play music of the user's choice. Some alarm clocks can set multiple alarms. Many alarm clocks have radio receivers that can be set to start playing at specified times, and are known as clock radios.

Alarm clocks are also used in mobile phones, watches, and computers. A classic analog alarm clock has an extra hand or inset dial that is used to specify the time at which the alarm will ring. To turn off the sound or light, a button or handle on the clock is pressed most clocks automatically turn off the alarm if left unattended long enough. Some have sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone who is deeply asleep, which causes tiredness, even if the person has had adequate sleep. Most use sound some use light or vibration. The primary function of these clocks is to awaken people from their night's sleep or short naps they are sometimes used for other reminders as well.

Traditional wind-up (keywound), mechanical, spring-driven alarm clockĪn alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time.

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