What is an alarm clock?

An alarm clock refers to a clock that signals people at a specific time. The main function of the alarm clock is to wake up the sleeping person or remind other things. Most alarm clocks use sound as a signal, and some use light, vibration or smell. Press the button on the alarm clock, the signal will stop, and some alarm clocks will stop automatically after a certain period of time. Some mobile phones and watches also have alarm clock function.