A timer is a specialized type of clock for measuring time intervals. By function timers can be categorized to two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer or a countdown timer. A simple example for this type is an hourglass. By working method timers have two main groups: Hardware and Software timers. Some timers sound an audible indication that the time interval has expired. Time switches, timing mechanisms which activate a switch, are sometimes also called “timers”.