Metal Detectors



Gold Detector Circuit


This circuit really is the simplest inducer balancing metal detector (IB, Inductlon Balance) that can be built.

The LB metal detection method has a satisfactory depth of penetration and good distinction between

iron-based and noble metallic objects. Several metal detectors are available in the market, but their price

is often prohibitive for seekers looking for buried treasures. This detector carefully constructed

and properly configured to accurately detect a 15mm diameter brass coil 70mm in air or a coin of 25mm diameter at 120mm.

Even at a distance of 25% more, the detector will give unclear signs of coinage.

Of course the ability to locate coins in the soil depends on the conditions of the ground between

the treasure and the search head, with the ideal material being the dry sand and worse the mud and,

of course, the size of the object.

find out more about how gold detector work 


The set of electronics required for the construction is relatively simple and is based on conventional components,

most of which may already be on the warehouse.

The electronic part consists of a pulse generator (transmitter) and the corresponding one-way

receiver where two coils are used as a means of processing.

The transmitter develops around the IC1. The (low power) CMOS 555 generates a square wave output signal

with a pulse ratio of nearly 50% and a frequency of about 700Hz. With 555 in unstable operation, the output frequency

is determined by the R4, R3 and C3 components. The output pulse is applied to the L1 coil through the R8 – C4 array,

in which the electrolytic capacitor prevents DC from passing through the coil and the resistor protects the output stage within the 555.

The pulse fronts generated by the 555 stimulate the coil and cause intermittent oscillations at the same frequency of approximately 10kHz.

Prior to the receiver unit (IC2) there is a simple yet parallel pre-amplification step which is based on the IC3 booster amplifier

and amplifies the signal received at the receiver coil (RX) via the components C11 and R9.


The RX and TX coils are in a very sensitive coupling so that the presence of some metal disrupts their coupling

and along with this carefully set the ‘calm’ of the threshold detector. The two coils are of the same size

and have a partial overlap. This overlay gives the coil the ability to collect both a positive and a negative (inverted) percentage

of the magnetic field generated by the TX coil. Due to the careful balancing of the coils, the positive and the negative signals

are mutually eliminated, resulting in the coil displaying at the output (theoretical) zero signal. The situation is called ‘zero’.

Nevertheless, due to practical limitations, there is a small waste stream. Once the sensitive equilibrium between the fields

is disturbed by the presence of a metal object (which will absorb energy from the magnetic field), the coil RX will begin

to provide the output with a larger current causing the electrical threshold set in IC2 to be exceeded and the Buzzer

will start sounding In practice, the best tuning of the detector is achieved at the limit where the metallic object is absent,

the passive piezo-electric buzzer produces a mild whistling sound. With this setting, when a metal object enters the field

the sound level will increase significantly. Setting the ‘zero’ point of the coils is particularly important.


Fixing the components on the board is not expected to pose any problems because the available space

is comfortable and secondly all the components used have terminals. Beginners should pay close attention

to the correspondence between the parts list and the layout outline printed on the board. The axes of the

potentiometers are passed through the board. This fixation mode was chosen to facilitate the fitting of the appropriate

knobs on the shafts when they are cut to the appropriate length depending on the box that will accommodate the circuit.

Because we are dealing with a relatively sensitive circuit, we recommend the box to be metallic.

In this way it is also easy to ground the potentiometer housing via the fixing nuts.


The two coils are identical. If it is not difficult to buy them, we can use enameled copper wire of 0.26mm.

With which we will make 100 right-handed spirals in a circular form of 15cm diameter.

The diameter of the wire is not critical and anything between 0.2mm and 0.3mm is OK.

Then we temporarily fix the thread with pieces of insulating tape which we pass underneath and fasten from above.

We also manufacture a similar coil, and once we finish, we tightly wrap the coils across the periphery with an insulating tape.

Then we need to add to each coil a Faraday cage.

Starting from the bottom of the duct, wrap the foil around the circumference of the coil so that no part

of the insulating tape is visible under the aluminum but the aluminum foil should not complete a full circle of 360 degrees.

Leave a small gap before we get to the point where we started (about 10mm) so that the two ends of the aluminum

foil do not come in contact, otherwise we will have a short-circuited winding around the coil which will introduce an awful amount of unwanted noise.

We do the same job on the second coil. Then connect each coil with a good quality symmetrical shielded

microphone cable where the Faraday cage is connected to the cable shield. Here we need attention

to the choice of cable, it does not serve to use a stereo microphone shielded cable, as it may cause interference between the coils.


The construction of the detection head has not yet been completed, some components are already beginning to interact

with the circuit settings. Before we start fixing the coils, it is necessary to have a complete and functional board.

The coil support  made on a hard non-metallic base, with the help of plastic Resin. As a base we can use any surface

of suitable size whose walls made of wooden pins of 5 mm in diameter perpendicularly glued.

Note that these walls should be leakproof (do not leave Resin), while we should not use any metal.


At this point the voltages within the coil RX are zero and we continue to increase the gain slightly

and moving the coils slightly. We repeat the process several times, increasing the gain every time.

The higher the gain, always having the ability to locate the zero point, the more reliable the detector will be.

Attention always drives the coils from full contact to full separation. If we set the system upside down,

once a metal object detected, the signal level within the coils will initially fall through zero

and then rise again to reach the point where the buzzer will start to sound.

At the operating principle level there is no problem, only the detector will be very unconscious.


The images illustrate an alternative method for implementing the search head,

based on the use of plexiglas. The coils TX and RX embedded in grooves drawn

at the ends of separate plexiglass semicircles.

A third piece of plexiglass (this time square) serves to support the semicircle on it,

connects to the PVC tube and still allows adjustment of the coils.

The last action achieved with the help of plastic screws, since this construction

completed and mechanically adjusted, and it must swim in Resin so as to ensure the necessary durability and durability.


as well as by the temperature and voltages, so the continuous adjustments of P1 and P2 are inevitable.

A part of the extraordinary simplicity of the circuit is the partial slip that it presents,

which although not excessive, necessitates a relatively regular adjustment of the detector.

At the center of the search head, the rejection of the circuit in the iron is too high,

so when one acquires the necessary familiarity with the detector, practically he is able to exclude the iron.

This is an awesome advantage for those who use the detector to find gold coins or noble metals.



R1 = 2k7
R2 = 470O
R3, RS, R6 = 100 kO
R4 = 4k7
R7 = 1 kO
R8, R9 = 100O
P1 = 50KO potentiometer
P2 = 2K5 potentiometer


C1 = 100µF / 25V electrolytic
C2 = 47µF / 10V electrolytic
C3 = 10nF
C4, C6 = 10µF / 10V electrolytic
C7-C11 = 100nF


D1 1N4001
IC1, IC2 = 555C, TLC555, 7555 (CMOS)
IC3 = TL071CP, TL081CP
IC4 = LM317 (CT220)


K1, K2 = terminal 3-pin terminal block, 5mm terminal block
BZ1 = passive (ac) piezoelectric buzzer
PC1, PC2 = board studs
12V battery or 8-cell AA battery pack
Reinforced copper wire diameter 0.2-0.3mm 2×50 meters
Box: (109 x 58 x 25 mm) 5m symmetrical
Shielded microphone cable (two lines with common shielding)





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