This is the 5585th most frequent German word.
bewegungsmelder
Motion detector.
In this sentence, 'Bewegungsmelder' is used to refer to a device that detects movement.
Der Bewegungsmelder reagiert bei jedem Schritt.
The motion detector reacts to every step.
Here, 'Bewegungsmelder' is used to specifically describe the type of device installed for monitoring movement.
Ich habe einen neuen Bewegungsmelder für die Garage installiert.
I installed a new motion detector for the garage.
In this case, 'Bewegungsmelder' implies its use as part of a security system monitoring movements.