Patent ID: 7830532

Claim:
A door/gate monitoring device having a sensor device comprising: a first source for emitting electromagnetic radiation; a receiver for receiving electromagnetic radiation; and a control device for selecting a first frequency for the electromagnetic radiation to be emitted from the source for emitting electromagnetic radiation; wherein, before emitting electromagnetic radiation, the sensor device determines whether signals are present that would interfere with the reception of the reflected electromagnetic radiation in a first frequency range that includes the first frequency, when signals are not present that would interfere with the reception of the reflected electromagnetic radiation in the first frequency range, the sensor device causes the first source for emitting electromagnetic radiation to emit radiation at the first frequency and determines a distance covered by the electromagnetic radiation emitted by evaluating reflected electromagnetic radiation from a reflection surface of an object received at the receiver for receiving electromagnetic radiation; when signals are present that would interfere with the reception of the reflected electromagnetic radiation in the first frequency range, the sensor device changes the selected frequency for the electromagnetic radiation to be emitted from the first source for emitting electromagnetic radiation to a second frequency and determines whether signals are present that would interfere with the reception of the reflected electromagnetic radiation in a second frequency range that includes the second frequency, and when signals are not present that would interfere with the reception of the reflected electromagnetic radiation in the second frequency range, the sensor device causes the first source for emitting electromagnetic radiation to emit radiation at the second frequency and determine a distance covered by the electromagnetic radiation by evaluating reflected electromagnetic radiation from a reflection surface of an object received at the receiver for receiving electromagnetic radiation.