Are you referring to a microprocessor in a computer? There are USB readers that could be utilized to secure computer access. When a person is seated at the computer wearing an RFID-enabled ID badge, an interrogator would pick up the signal and the computer would function normally. If that individual stood and walked away, the reader would no longer detect the tag, and would thus put the computer to sleep, requiring a password or the RFID badge to re-awaken it.
I do not believe there is an off-the-shelf system available, but I have heard of government agencies that have implemented such a system in order to prevent unauthorized access to a computer while a person is away from the machine. As for read range, that depends on the particular reader and tag used. A low-frequency (LF) or high-frequency (HF) system would work well, because the range would be two to three feet—approximately what you would want. You could also use an ultrahigh-frequency (UHF) system, turning down the antenna output power to read tags only a few feet away.
—Mark Roberti, Editor, RFID Journal
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