Functional principle:
Advancing technical development has made it possible to offer very small and in-
expensive radar sensor modules. This enables the construction of innovative motion
detectors.
They work according to the so-called “Doppler effect” (Christian Doppler, 1803 - 1853),
which implies that the frequency of an emitted signal varies as the source of the
signal approaches or moves away relative to a receiver. We encounter the Doppler
effect in everyday life, for example in traffic. If a honking car drives by at a high rate
of speed, then the pitch (=frequency) of the horn seems higher as the car approaches
and lower as the car moves away.
This phenomenon is also true for electromagnetic waves. Thus, electromagnetic waves
in the cm range are emitted as for instance in road traffic radar. If these waves are
reflected by a moving object (i.e. an automobile), then their frequency has changed
proportional to the speed of the object. Using this variation in frequency it is possible
to calculate the speed. In the same manner the frequency to be received varies, if
a person moves in the sensing range of the radar sensor. This variation in frequency
is analyzed by the sensor, which supplies a corresponding output signal, which can
further be processed by a following electric circuit. The working frequency of the
sensor is approx. 2.45 GHz. The low transmitter power (typ. 8 dBm EIRP) poses no
health risk whatsoever..
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