Monday, March 30, 2009

BB Metal Detector

One of my better-known electronics feats was the design of the world’s simplest self-contained metal detector (see diagram). This is based on the Beat Balance (BB) principle -- also one of my inventions. The design shown here, despite using just five components (not counting on-off switch), offers sensitivity way beyond that of Beat Frequency Operation (BFO). If well adjusted, it will pick up an old English penny at 150mm (6”). Briefly, BB uses two search oscillators, and each search coil modifies the frequency of the ad­ja­cent oscillator through inductive coup­ling. This sets BB apart from BFO, which uses a single search oscillator and a reference oscillator. It also sets it apart from Induction Balance (IB) which uses one "transmit" oscillator and a "receive" circuit. OBSERVATION: This design is bas­ed on two extremely sim­ple in­ver­­ter oscil­lators (also my original design). Their outputs are then mixed, creating an au­dible beat frequency in crystal earpiece X1 (note that this is not the same as a high impedance ear­piece). The circuit is tuned with VC1, an ordinary AM tuner. For details on the construction of search coils, see Attach the coils to the circuit via balanced (figure-8) mic­ro­phone cable, with the cable's screens going to the coils’ Fara­day shields and -12V (negative). The coils may be bound to a rigid plate with cable ties, and should overlap each other by up to one-third (experiment for best sensitivity -- this requires meticulous movement of the coils before binding). It makes some 20% dif­fer­ence to the metal detector’s sensi­tivity “which way up” coil L2 is. Flip L2 to find the side which offers best per­form­ance. An improvement could be made to the stability and sensitivity of the circuit by using a regulated power supply. You may enlarge the diagram by clicking on it. Once again ... happy hunt­ing!
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (


Unknown said...

can regular headphones be used instead of crystal earpiece?

Thomas O. Scarborough said...

Yes and no. A standard earpiece is say 16 Ohms, while a crystal earpiece is infinite Ohms. So a standard earpiece will draw current, and probably needs a series capacitor to block DC (say 10 microfarad). I doubt that a standard earpiece will give the same performance as a crystal earpiece. One could also try a piezo sounder without internal electronics. A crystal earpiece is best, but others could be used temporarily if one can't find one. See also my digital design at which has a stronger output for the earpiece.

Unknown said...

thank you for quick answer, i can salvege sounder and give it a try.