Metal Detector Circuit

Metal detectors can be classified based on their operating principle into these three categories: BFO, TR/IB, and PI. Each of these methods has advantages as well as disadvantages. The ideal metal detector (don’t look for it because it doesn’t exist). You must take advantage of all the advantages of all methods, eliminating their disadvantages. However, the detector must be sensitive enough to give some indication of the type of metal detected. The detector shown here belongs to the TR / IB class of detectors, for this reason its head consists of two inductions. As will be seen later, the entire construction is based on the combination of a variable L oscillator and a detector.

Types of metal detectors

• 1) BFO (Beal Freqency Oscillalor) In this category of detectors, the probe induction is part of an oscillator whose variable output frequency contributes to a constant frequency generated by a second oscillator. The result of this contribution is a frequency in the acoustic region. As soon as the detection head approaches a metallic object, the variable oscillator causes a change in the interference frequency, which can be perceived by sound or in any other way. BFO metal detectors are relatively inexpensive and easy to use.
• 2) TR / IB (Transmission-Induction / Balance) The operating principle of these sensors is based on mutual induction between the transmission and reception coils. Once a metallic object is close to the two coils, the coupling coefficient changes, resulting in a change in the output level of the oscillator.
• 3) PI (Pulsed Induction) Here a continuous pulse train is output, producing reverberation signals that are examined for their shape and amplitude. This can reveal the presence of metals within the area covered by the transmitter.

Magnetic properties

Each metal object can cause changes in the inductance of a coil as well as the coupling coefficient of two coils. This type of effect, which can be positive or negative, depends on the relative permeability (μ) of the metal in question. Here it should be mentioned that the materials are classified (Table 1) as paramagnetic, diamagnetic and ferromagnetic.

Determining the composition of an object based on the measurement (μ) is very difficult. However, due to significant differences in measurement (μ), a distinction can be made between paramagnetic and diamagnetic materials. on one side and ferromagnetic on the other.

By placing a conductive material within a changing magnetic field, various currents are induced in it. The intensity of these currents depends on the shape and size of the metallic object, as well as the resistivity of the material or materials of which it is composed. At a level and with a sufficiently large metal plate, the intensity of the currents can obtain large values. However, if we create slots on the same plate, the current intensity will decrease. Other factors that determine the intensity of the currents are the location of the material within the magnetic field (ie, the number of dynamic lines that intersect it) and the surface composition of the earth.

From all this, one can understand the difficulty of determining the composition of the buried material. Use of a single measurement method.

Circuit description

In the detector circuit transistor T1 acts as a self-modulating oscillator. This means that a low signal and a high frequency signal are produced which is very similar to the AM waveform in the image below.

 

The slope of the positive front of this composite signal is greater than that of the negative side. The switching of the oscillator between the two states (open / closed) is carried out with the help of D1, C1 and R1.The capacitor C1 during oscillation is charged through the diode D1 until its voltage cuts off T1. At this point the oscillation stops and Cl begins to discharge through R1, until its bias allows T1 to reopen.

The transmitter coils L1, L2 and L3 are connected between the base and collector of T1. In practice, these inductions are arranged in such a way as to neutralize parasitic capacitances that can affect the stability of the oscillator.

Capacitor C5 is placed on the head to avoid the influence of the parasitic wiring capacitance between the head and the detector on the stability of the oscillator.

Coils L4 and L5 form the coupling loop and are also located in the detector head. The depth signal from L4-L5 can be compensated by capacitor C6 which also cancels the detector output by aligning the coils and receiving.

With P2 you make the main sensitivity selection and with P1 you make the very fine adjustment of the detector sensitivity. Diode D2 is used to suppress any negative voltage that may occur at the inverting input of IC1. The operation of the detector is very simple. As soon as the rectified input signal (diode D2) exceeds the threshold voltage of the non-inverting input of the comparator, the IC will change state. Thus, the output, which is an open collector, takes the logic value (0) and activates the transistor T2 which drives the loudspeaker. The pitch of the sounding note depends on the signal level obtained from the receiving coils L4 – L5 (the dotted horizontal line in the drawing below).

 

As the intensity of the received signal varies, the length of time that the signal exceeds the threshold changes. This results in a change in the pitch of the (perceived) sound with each detection of a metallic object.

Through D3, R7 and C12, the output voltage of T2 is converted to a negative feedback voltage for the comparator. This creates an AGC (Automatic Gain Adjustment) circuit that compensates for large changes in input level. Moving coil M1 gives a visual indication of signal strength. With push button S2, you can check the batteries.

Building

The final performance of the detector depends largely on the good construction of the detector head. The coils will be mechanically supported, on a plastic sheet, in dimensions and shape as the previous drawing.

If you use wood as a support material (better not to try), the head will be sensitive to changes in ambient humidity and will not be able to reset the detector. Using a cutting tool, make a notch in each sheet with a width of 5 mm and a depth of 10 mm.

The coils will be made from 0.3mm varnished copper wire (30 SWB) following the procedure: Seal the start of the first wrap at point A on sheet 1. Passing through the notch around the side of the sheet , for coil L1). Stop at point A and take a shot by twisting the wire to the length of 10cm, which will stick to the surface of the blade. The remaining border, for now, just forget about it. The L3 coil boot, which will be built by winding 4 left handed coils around the L1 coil, will connect to the socket we made.

To wrap L3 you will start from point A. ending back at the same point. The free end of L3 will stick to the blade. Continue building the winding L2 starting from the wire that we left free after winding L1. L2 will consist of 22 clockwise turns that will pass through the notch, starting from point A and ending at the same point. Glue the edge that will go over the sheet.

Construction of the receiver coils will be done on sheet 2 following the procedure below. Starting from point B and ending at the same point, measure 36 turns clockwise for coil L4. Take a shot in the same way as with L1 and stick it together with the beginning of the winding on the surface of sheet 2. Proceed with the same wire by winding the coil L5 consisting of 36 turns clockwise. Stop at point B and glue the edge of the last wire to the sheet. Carefully check all wire ends as well as coil contacts. Secure the capacitors C5 and C7 on the leaves and connect them with the corresponding cables.

Using a cutter and file, cut a slot in sheet 2 and drill holes in both sheets to fit. The screws and nuts that will be used for the support must be made of plastic material.

The construction of the remaining mechanical parts is left to your personal taste. The scanning head and electronic box can be attached to a piece of wood or a PVC pipe. Prefer PVC because it can hide the wires between the head and the detector.

The assembly of the parts with the help of the plate is a routine case. On the front of the box it will include electronics. Five sets of S1, S2, C6, P1 and P2 are shown. However, a sixth can be added as shown below. Connections between the resonant circuits of the head and the detector must be made with shielded cable.

The detector head itself can be placed in a suitably selected plastic box. The various empty spaces between the plastic case and the head can be filled with polyurethane or epoxy resin while creating a compact construction.

Configuration and adjustment

First, the plastic sheets are adjusted to the greatest distance allowed by the regulating plastic screw. Jumpers A and B should not be placed on the board while all sliders should be in the center of the path. Give voltage to the detector circuit and find out with the help of regulators P1 and P2 if a sound can be generated. When this work is about to be done, there should be no metal around the detector head.

Carefully start to align the two blades until they reach a position where the intensity of the speaker is reduced. Increase the distance between the two blades to approximately 0.5mm and tighten the adjusting screw. At this point you can fit the coil system into the detector head and seal it with some suitable material (mentioned above). Install jumper A and see if you can set a blank on the detector output with setting C6. If setup fails, put jumper B. If setup fails again, put a 470pf capacitor parallel to C6. If the problem is not resolved permanently, your only solution is to build a new scanhead.

Supply the circuit with a stabilized voltage of 9V and adjust the sensitivity slider so that the detector does not produce any sound. Press S2 and adjust P4 to fully deflect the needle M1. Reduce the supply voltage to 7V and mark the new pointer position in red. With the P3 trimmer you can adjust the sensitivity as you like.

One final note on the oscillator. The output can beep (frequency 100-150 Hz). You can remove it by putting a 50K pot (the sixth slider) in line with R1.

Use of detector

For those who will use the detector for the first time, it is best to try the effect of the C6 settings. The sensitivity of the detector is higher when the sound produced by the loudspeaker is too low. By turning C6 left or right from the zero point, you can determine if the detected material is ferromagnetic or paramagnetic or diamagnetic. However, experience is also the most important factor in the proper operation of the detector, which under the right circumstances can discriminate a small coin from a metallic dirt floor to a depth of 20 cm. Happy gold hunting 😉

Components List

Resistors (5% tolerance):