A door that unlocks when you walk up to it - a company building that idea just raised $38 million

August, the company behind the smart lock of the same name, announced Wednesday that it raised $38 million in a round led by Bessemer Venture Partners.

The August lock replaces your regular door lock and you control it with a smartphone app. It uses Bluetooth to detect when you (carrying your phone) approach the door and unlocks automatically. You can also assign digital keys to guests using the app so they can enter your home when you're not around.

Reviews of the August lock, which launched last fall, were mixed.

New York Times columnist Farhad Manjoo wasn't a fan, and wrote: "At $250, the August lock is more of a chic bauble than a useful necessity." Re/code's Walt Mossberg found the device more useful: "I liked it and can recommend it, especially for folks who often have to give keys to strangers," he wrote.

The ultimate vision for August is to replace your keys with your smartphone and eventually build a system of other "smart home" gizmos. It's one of several companies experimenting with the "internet of things" (IoT) trend, which means connecting everyday appliances and objects to the internet to make them more useful.

Everyone from Google to Apple to Samsung have been dabbling in IoT recently, but so far no one has made a must-have gadget or proven the concept can go mainstream.

The company has raised a total of $50 million. Comcast Ventures and chipmaker Qualcomm also participated in the round.