despertador
alarm clock
despertar
to wake up
+
-dor
person or thing that performs an action
A despertador is an alarm clock, which can be thought of as a thing (-dor1) that wakes you up0 (despertar0).
noun
- alarm clockAn alarm clock or device that wakes someone upPongo el despertador a las 7 de la mañana. // I set the alarm clock for 7 in the morning.