What is it?

The HC-SR04 is a commonly available low cost ultrasonic range sensor. It is dumber than some, which is why it is cheaper. Your Arduino has to handle the timing and calculations.

I'm selling these in packs of 4. Also optionally add 200mm/8" cables.

See also my Trimount bracket sets to mount up to 12 of these on the corners of your robot.

Now available are the 3.3V HC-SR04P. These are considerably more expensive and require desoldering/replacement of the headers for them to fit on my Trimount brackets. Replacement headers are included. Unless your project has no access to a 5V supply, these are not for you. When operated at 3.3V they also have shorter range. My OctoSonar modules can operate the 5V sensors from a 3.3V controller, with a separate 5V supply for the sensors.

Why are you selling these?

I've created the Octosonar module, its big brother the OctosonarX2 and the Octosonar library to handle up to 16 of these using the I2C bus and one hardware interrupt pin. I'm stocking the HC-SR04 as well so buyers can be sure that they will work with the Octosonar.

Why buy these here?

Convenience, mostly. However not all HC-SR04 are the same - there are at least three different clone designs out there.

One variety locks the Echo pin high if it does not get a return signal. See this thread on the Arduino forum for details. The original Octosonar - which used the SonarI2C library - would not work reliably with these, and they also cause problems with the standard Ping library. The OctoSonar was redesigned to work around this problem and now works reliably will all clones I have encountered.

I test each sensor individually before bagging them up and discard any bad ones.

I have added the HC-SR04P as an option. While the standard HC-SR04 needs a 5V supply, the HC-SR04P will operate at 3.3V, though with a lower range. These units are more expensive. They also have the headers soldered on the other side from the 5V units so will not fit on my Trimount brackets. These packs include replacement headers so if you know your desoldering skills you can swap out the headers and make them fit. Note that the OctoSonar can talk to a 3.3V controller and take a separate 5V supply for the sensors, so you only need to bother with the HC-SR04P if your project has no access to a 5V line. See the pictures above for the correct placement of the headers - on the back with the insulator flush to the board.

For reference I buy these in batches from banggood.com. If you are not in a hurry, you can too.

Pedantic Comment

Yes, I know the picture only shows two sensors. I used to sell them in packs of two and I don't have an updated picture. Cross your eyes. See? Problem solved.

Also the video below is out of date and shows the earlier board and the link to the old version of the software. See the Octosonar library for the correct code.