Metal simple and powerful detector

Metal simple and powerful detector

My daughter got into her head to want to use a metal detector to search for ancient artifacts, where I live there are many areas to explore, the possibility of discovery are very low but, with a little luck you can find exhibits ranging from Etruscan times up to World War II. Not wanting to spend on a desire that could last less than a month before, find myself with a metal detector in the house that takes dust I decided to build one I. I appreciate this hobby, because it relaxes and leaves the mind free from everyday problems. Currently, the commercial metal detectors are equipped with microprocessors, This facilitates the processing of information, making them faster and more accurate identification and localization of metals.

But there are several detectors unprocessed who hit among those who want to realize their metal detector, including the Surfmaster PI and this

all of PI technology (Pulse Induced). But even these detectors are considered by many difficult to achieve, This is why I decided to design a version metal easiest detector, but with PI technology.

Although many engineers do not agree, a PI detector can discriminate if well designed, different metals, It depends on the sampling frequency.

Metal Detector Amilcar PI

My metal detector with false modesty that I called Amilcar Pi is easy to build it is already pre-adjusted for maximum efficiency and does not contain components difficult to find, and what not least is also very low cost.

The metal detector is composed of two main units, a transmitter that uses the generator of narrow pulses to be 150us, retrieved from the literature on the network is the most efficient solution and to lower consumption. These pulses amplified by a mosfet IRF740 piloting an emitter coil. The same coil also acts as a receiver and its main component is the LM324, a quadruple operational amplifier.

The starting point of this handset is inspired by a metal detector originally developed in Russia. In my there is a reworking of the same ideas with European components and, while I was there optimize the oscillator part to save mA always useful for battery-powered devices. The result was a very similar to Surfmaster PI detector in its operating principle, but much simpler and “quasi” the same efficiency.

The electronic circuit works so: It starts with a square wave oscillator which rotates on an oscillating operational at a frequency of about 130Hz with on-off ratio very unbalanced while 7,3mS Ton = Toff = 0,15mS.

The output is inverted by Q2 which serves to drive the mosfet, the scelda of R6 and R7 is designed to compensate for the fact that the operational amplifier output never reaches the supply voltage, They are typically 1.5V “eaten” by the operational, the divider serves precisely to ensure that when the output is at one of the transistor Vbe is below the conduction threshold to zero while when it is able to take it into saturation. The resistors R8 and R9 ensure that the mosfet has rising edges and very rapid descent to avoid unnecessary heat loss and give the maximum power to the transmitter coil. R11 together with D3 and D4 act with C4 ago detector for the extra voltage induced when stopping driving the mosfet. It appears obvious that esa will be much higher as much as a metallic element will be found in the magnetic circuit of the coil. R12, R13 and R14 securing the comparator threshold levels for signal detection so as not to have anything in headphones if not in the presence of a metal object. R15, r16, The potentiometer R17 and C3 serve to filter out high frequencies and simultaneously regulate the volume of the headphones. C7, R18 and the last transistor amplifies the signal picked up at an optimum level for headphones.

The consumption at idle settles on 20mA, these expedients guarantee an acceptable duration of 9V battery, But nothing prohibits to use a lithium battery pack 2S or 3S for a full day of uninterrupted research.

The metal detector coil

The construction of the metal detector coil is simple, the coil is composed of 25 turns of wire from 0,5 mm with a diameter of 19 cm.Con this setting the sensitivity to a coin 25 mm is 20 centimeters. Other diameters may be tested for an optimal result, the table below by some indications regarding the number depending on the diameter coils to always have an inductance of approximately 400uH

coil Diameter Number of turns inductance
100 mm 42 402uH
120mm 36 405uH
150 mm 31 394uH
175 mm 28 387uH
190 mm 25 401uH
200 mm 26 406uH
250 mm 22 380uH
300 mm 20 390uH
400 mm 17 396uH
500 mm 15 400uH

The base dual electronics face is 44mm x 58mm but it is not critical and you can achieve it however you like

I by the time I left everything in the hands of my daughter, at least until it will not be fed up of the new “toy” and shall put on a mobile gathering dust.

Amilcare Greetings

40 replies
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  1. tech
    tech says:

    Ciao, using the formula for the inductance of a solenoid, with your data: 25 coils with wire diameter 0.5mm and coil diameter 19 cm assuming that the coils are superimposed one all’ the other gives me an inductance of 0.00178 H and not 400 microH (400*10^-6 H). What formula did you use?

  2. walter
    walter says:

    good morning Amilcare, I'm making the metal detector, I have two questions:
    1) i developed the PCB using 3 single opamps to simplify connections but I don't know which opamps to use. I have TL071s, can be fine?
    2) since the pcb was in a board 5 x 7 cm I would also like to use it as a wire finder placed under plaster and therefore in addition to the normal reel I will also try to wind a reel with a smaller diameter: 5 O 6 cm maximum You think it might be a good idea?
    Thank you

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