# How to read the datasheet of an ultrasonic sensor?

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.

• Could you provide more information? Power supply voltage range; IP protection level (IP65, IP67 ...); response time; current consumption; do I need control inputs? About output... analog, digital or both outputs? Commented Feb 28, 2018 at 20:04
• IP 65 is enough. For the output I prefer digital. I don't have constraint on other parameters. Commented Feb 28, 2018 at 20:06

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

• I think you've left out some detail to explain why this limits the minimum sensing range. Commented Feb 28, 2018 at 20:07
• @Transistor Just like radar: the receiver is de-sensitized by a huge transmitted pulse, and some time is required before the receiver achieves full sensitivity. Have added an edit. Commented Feb 28, 2018 at 20:34
• That fixes it. There was a similar question recently and the same issues were quoted. It's probably also worth pointing out that at standard temperature and pressure that the sound travels 1 cm in about 27 µs which is roughly equal to the wavelength at 40 kHz. A pulse of 10 x 40 kHz cycles already takes you out to 10 cm. Commented Feb 28, 2018 at 21:22
• @Transistor For very short ranges, the radar technique of transmitting short pulses and looking for reflected pulses fails, as you've pointed out. Perhaps better to transmit continuously, and look for phase variations of the received reflected wave. Have had success with this method at short range. Commented Mar 1, 2018 at 0:40
• What if I use one as receiver and one for trasmitter? Commented Mar 1, 2018 at 14:00