I want to use ultrasonic sensor for a personal project. I need a waterproof sensor like this but I cannot figure out how to calculate the measurement range. Sometimes on these sensors is written that the minimum range starts from about 25cm.
For my purposes I need a sensor which is able to measure from about 10cm. How to do it? HC-SR04 is very good for my idea, but it isn't waterproof.
Google can't help me this time.
Best Answer
A big problem with minimum range is that these transducers have significant Q, as demonstrated by the bandwidth of 1-2 kHz. This means that the transmitter pulse rises and decays slowly. And the receiver pulse rises and decays slowly.
Edit: The slow response of the envelope means that a reflected short-range pulse enters the receiver before the transmitter has entirely decayed. The transmitter inevitably sends a direct signal to an adjacent receiver, and the much smaller reflected pulse may be hidden by the larger direct-path signal. The 2cm minimum range for HC-SR04 is optimistic, especially for small irregular reflecting objects. To improve the minimum range, rise and decay times should be shortened.
A simulation of a similar bare 40 kHz transducer yielded a slow envelope rise (top diagram).
The transducer bandwidth can be broadened by resonating out the large parallel transducer capacitance of the piezo element (C3):
simulate this circuit – Schematic created using CircuitLab
For example, a transmitter circuit (partial) could look like this, with the pulse source lasting about 150 microseconds. The transmitted amplitude should die away as quickly as it rises on its leading edge:
simulate this circuit