The Fairy Tracker is a device that detects fairies (even in their disguise).
This radio-like device, allows the user to detect faries even in disguise. In the episode Transparents, it detected Cosmo and Wanda even in a human disguise. It features a solar-powered antenna and a small screen. It is used by Crocker in multiple episodes.
This is one of Crocker's many fairy-detecting/capturing inventions.