Metal detectors are obviously the number one tool of the treasure hunter, particularly of the hobbiest THer. The three primary types are the beat frequency oscillator (BFO), induction balance (IB), and the pulse induction (PI). The popular transmit-receive (TR) and very low frequency (VLF) detectors are generally categorized as IB designs.

Theory BFO Theory Electronic Metal Locators Multilayer Air-cored Coils Projects/Designs PLL Detector (Kamil Kraus, Electronics World) Basic BFO Detector (Rachel and Steve Hageman, EDN) Jetco Treasure Hawk Metal Detectors (Gavin Cheeseman, Electronics and Beyond) Magnum IB Detector (Andy Flind, Practical Electronics) Coinshooter IB Detector (William Lahr, Popular Electronics) Buccaneer IB Detector (Andy Flind, Everyday Electronics) Phaser TR Detector (Practical Electronics) Shadow IB Detector (ETI) Two-box IB Detector IB Detector (Andrei Chtchedrine) Basic IB Detector (ETI) Pulse Induction (J.A. Corbyn, Wireless World) GoldPic Pulse Induction (Trevor Hill) Pulse Induction (ProScan) Microcontroller Pulse Induction (Mark Stuart, EPE) Twin Loop Treasure Seeker (Robert & David Crone, ETI) Coil Winder Other Links How Metal Detectors Work DetectKnowledgy ProScan Eric Foster's Tech Forum Detector Repair