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 http://thomasscarborough.blogspot.com/2008/05/bfo-metal-detector-2.html. 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 (http://thomasscarborough.blogspot.com).
3 comments:
can regular headphones be used instead of crystal earpiece?
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
http://thomasscarborough.blogspot.com/2013/06/bb-metal-detector.html which has a stronger output for the earpiece.
thank you for quick answer, i can salvege sounder and give it a try.
Post a Comment