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Offline ghostloccTopic starter
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« on: January 30, 2010, 06:37:29 pm »
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hi everyone im new this is my first post. i am looking to build a metal detector for my school senior project that is due a month from now. i need help on the materials i need and some instructions because i have literally looke don every website and found nothing that can help. ive seen the easytreasure one with the wooden stick but i cant find any instructions or materials for that. i would really appreciate it if anyone can help me on my project by giving me some materials or easier metal detectors i can build under the little time that i have. the due date is march 15 and im a little stressed out because i think i wont get it done in enough time. please help anyone?? it will be highley apprieciated. thanks

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Offline gambol1
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« Reply #1 on: January 30, 2010, 09:32:25 pm »
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ghostlocc, I may be able to help you on this one. Since your time is short you probably don't have time for a major build. It usually takes a month for me to get everything together. The following link

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http://www.freepatentsonline.com/3769575.pdf
has a PDF file of a detector that is ideal for a classroom demonstration. It works and it is as simple as you can get. You will need a DVM and a soldering iron. The coil I used was 3 inches in dia and had 40 turns.  I built this as a probe and it has a sensitivity of about 2 inches for a penny. Not much good in the field but as a demonstration of the principle it will work. It uses one tranistor in a hartley circuit and a cheap AM radio as a reciever. you tune the am radio to the oscillator frequency and and wear the radio around your waist. It works better if you keep the radio and the coil at the same distance at all times. If it doesn't work the first time it is probably because the oscillator frequency is wrong or the oscillator isn't working. Buy 2 transistors and pay especially close attention to the pin out. If the first doesn't work replace it. One note of caution. When you are working with an AM radio on the bench the 60 cycle buzz from the house wiring will drownd out the oscillator. I suggest you wrap a wire around the radio several times and then wrap the radio in aluminum foil to block the buzz. run the wire to a battery terminal of your detector. After you are sure the oscillator is working, you can remove the foil and wire. (stay away from cfl bulbs they are noisy) Good Luck


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Offline pulper
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« Reply #2 on: February 18, 2010, 10:09:37 am »
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Interesting and simple way to get started.

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Offline gambol1
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« Reply #3 on: February 18, 2010, 06:48:40 pm »
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Pulper, It is deceptively simple. The patend did not mention one important requirement necessary to make anything other than a "demonstration".  A BFO working at AM radio frequencies will detect anything. Your hand or wet sand or a wooden pencil. In order to get something to work in the field the BFO frequency has to be around 100 KC. (225 will work for wet sand).  The inductance of the detector coil has to be such that the detector oscillator is beating against  a low frequency spurious oscillation of the AM radio oscillator. A cheap radio produces a number of harmonics and spurious oscillations. Some are down around 255KHz when the radio is tuned around 550 kHz. Finding the right oscillation is a trick because they are weaker than the fundamental. But it works when you find one. Gambol

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Offline xavier
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« Reply #4 on: March 24, 2010, 04:36:12 am »
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So how did the project go?

Xavier

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So many questions so little time

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