Sentry
The sentry was an energy weapon that guarded a fountain in a cave on Lambda Paz, a desert moon of Pentarus III, until 2367.

The EM pattern given off by the sentry remained between 5 and 15 megahertz when the water fountain it protected was not threatened. When the fountain was approached, the sentry's frequency would rise to 40 megahertz. The sentry neutralized threats to the fountain by encasing them in electrically-deposited selenium fibers. 

In 2367, Dirgo, Captain Jean-Luc Picard, and Ensign Wesley Crusher found the fountain and the sentry while stranded on Lambda Paz. Crusher eventually found a way to deactivate the sentry using a modified tricorder, but not before it had killed Dirgo. ()

See also 
 Lysian sentry pod
 Mars sentry pod
 Sentry probe
 Species 116 sentry vessel
 Vaadwaur sentry satellite

Energy technology
Security technology