Elecfreaks HC-SR04, User Manual

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Attention:
   
The module is not suggested to connect directly to electric, if connected

electric, the GND terminal should be connected the module first, otherwise,
it will affect the normal work of the module.
When tested objects, the range of area is n

ot less than 0.5 square meters
and the plane requests as smooth as possible, otherwise ,it will affect the
results of measuring.
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