Wednesday, May 21, 2008

BFO Metal Detector [1]

Here's something completely different. I'm well known for my electronic designs, and here's one of the classics. It gives good performance and stability for a beat frequency operation (BFO) metal detector. BFO in itself, though, is not very sensitive -- yet it's easy and fun. This metal detector will pick up a 25 mm (1") diameter coin at 90 mm (3½"). The design is based on a simple inverter oscillator, which is attached to a Medium Wave radio aerial. When metal is brought near to coil L1, the frequency of the oscillator changes, bringing about a shift in the difference frequency in the MW radio speaker. Solder the circuit and take a wire to the MW radio aerial as shown. This should be screened, with the screen going to -6V (negative). Switch on both metal detector and radio, and tune the radio until a clear heterodyne (a whistle) is heard in the speaker (some heterodynes will work better than others). There will be a "band of silence" at the centre of most heterodynes (a zero beat frequency), and depending which side of this zero beat frequency is tuned in, the tone in the radio speaker will rise or fall at the presence of metal. It will also respond differently to ferrous and non-ferrous metals. OBSERVATION: Click on the diagram to blow it up. You can find instructions for the coil at http://thomasscarborough.blogspot.com/2008/05/bfo-metal-detector-2.html. A similar design reached the top of the charts at CircuitDB: http://www.circuitdb.com/. To follow the electronics trail on this blog, click on the "Electronics" category top left. Have you seen my electronics books? http://stores.lulu.com/store.php?fAcctID=980032
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough).

BFO Metal Detector [2]


Here are instructions for the search coil of the metal detector above. It requires 70 turns of 30 swg (22 awg) enamelled copper wire on a 120 mm (4¾") diameter former. Neither the winding of the coil nor the wire gauge are critical. Wind the coil around the former, and temporarily hold it together with stubs of insulating tape. Then bind it tightly with insulating tape all round. Scrape the enamel off the ends of the coil's enamelled copper wires. Then add a Faraday electrostatic shield, for stability. Prepare some long, thin strips of tin foil. Twist a 100 mm (4") length of bare wire around the coil, over the insulating tape. This provides electrical contact with the foil. Now wind the foil around the circumference of the coil -- beginning at the base of the bare wire -- yet the foil should not quite complete a full 360°. Leave a small gap as shown. Now tightly bind the whole Faraday shield with insulating tape. Attach the coil to IC1 by means of microphone cable, with the cable's screen(s) going to the Faraday shield at the one end, and to -6V (negative) at the other. The coil may be mounted on a rigid plate, and a suitable handle attached. OBSERVATION: Click on the diagram to blow it up. You could wire the coil directly to the circuit if you're in a hurry.

NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (http://thomasscarborough.blogspot.com).