The phrase "a simple device that" is correct and usable in written English.
It can be used when describing a device and its function or purpose in a straightforward manner.
Example: "A simple device that helps you measure temperature accurately can be found in most kitchens."
Alternatives: "an easy tool that" or "a basic instrument that".
The charge controller is a simple device that looks after the battery.
Or a simple device that measures blood sugar levels and your pulse.
He was, however, enthusiastic about his main purchase that night: a $24.99 Diaper Genie, a simple device that seals used diapers inside a "fragrant film".
The answer, he decided, was a simple device that would offer entertainment, making it something that even the world's poorest citizens might be willing to pay a sizable share -- perhaps more than 5percentt -- of their annual income to own.
Ms. Roberts sees the screen not so much as a sophisticated multimedia platform, but as a simple device that can lead visitors around a museum, highlight works not being exhibited and display text for the hearing impaired or the sponsors' corporate logos.
But if the founders of Bookscan have their way, the book industry will soon be transformed by a simple device that has long been available to other media industries: for a fee, publishers will be able to obtain access to precise and accurate sales information on every book that's published.
A physical key, for example, is a very simple device that allows a human to interface with a complex locking mechanism without needing to know how the latter works.
The modern push-button telephone handset provides a good example of a relatively simple device that has required a great deal of human-factors engineering.
By contrast, a file server may be a very simple device that runs an embedded server software such as network-attached storage (NAS), which is an appliance with disks that connects to a LAN and uses network file system (NFS) or similar to serve workstations.
For millions of consumers, though, the possibility of trading their old PCs for a slick, simple device that uses a Web browser to do just about everything they would want is increasingly close.
Initially, two major types of fission bombs were built: a relatively simple device that uses uranium-235 and a more complicated mechanism that uses plutonium-239 derived from uranium-238.
