Sentry

From Memory Alpha, the free Star Trek reference.

The sentry was an energy weapon guarding a fountain in a cave on Lambda Paz, a desert moon of Pentarus III.

The EM pattern given off by the sentry on one of the moons in the Pentarus system 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. Ensign Wesley Crusher eventually found a way to deactivate the sentry using a modified tricorder. (TNG: "Final Mission")