Hi all. We hope you already have an auxiliary miniature metal detector, which is used to accurately determine small metal objects in the ground? If not, then let’s fill in the gap. It also determines the location of metal pipes in the wall or reinforcing bars in the ceiling, which is important when drilling holes, so not only cop lovers will need this device. 

Inexpensive commercially available metal detectors and pinpointers, as well as similar devices, descriptions and diagrams of which can be easily found on the Radioschemes website , operate on the principle of detuning the LC circuit under the influence of metal approaching the probe coil. A small radius of action and a strong susceptibility to the influence of the soil is their main disadvantage. 

The presented device works on a different principle. It has two coils, one of which is a transmitter and the other is a receiver. Their mutual arrangement is such that the influence of the transmitting coil on the receiving coil is minimal. A metal object that is close to them will upset this balance, causing an increase in amplitude and a phase shift in the signal. This principle makes it possible to obtain a much greater detection range, as well as to minimize the effect of soil mineralization on the operation of the detector. 

SCHEME OF A DEVICE FOR SEARCHING FOR METALS

THE MAIN CHARACTERISTICS OF THE PINPOINTER

• operating mode – static, frequency 10 kHz 
• range – coin 5 cm to 6 cm, for large objects over 20 cm 
• does not respond to stones containing iron compounds 
• low power consumption (9V – 7mA) 
• audible alarm and LED
• probe shape suitable for immersion in the ground 
• low cost – assembled from readily available radioelements

The pinpointer is assembled on a small board: its dimensions are 85 by 17 mm.

If you want to do the SMD version, here is the PCB design for planar parts.

The search probe is of course the hardest thing to make. To assemble and adjust it, you need a frequency meter and an oscilloscope. During construction, most of the rules that are used in the execution of a conventional VLF probe are applied. 

DRAWING FOR MAKING A COIL

We wind all the coils in the same direction, marking the beginning with dots in the figure. After winding L1 and L3, we impregnate about 200 turns with cyanopane. We connect to the circuit, connect the power source and measure the frequency. If the value is about 10 kHz, then we are trying to pre-compensate – by moving L2 over L3, we set the signal (observing on the oscilloscope screen) on pin 1 of the LM324 to the lowest possible level. At this stage, the folded waveform may be distorted (significantly different in shape from a sinusoid). 

Now we select the capacitor Rx. To this end, we temporarily connect the winding of the receiving coil instead of the other and try to achieve resonance at a frequency slightly higher (about 200 Hz) than Tx. After that, we correct the compensation and immobilize the ends of the L2 wire with cyanopan. 

After inserting the winding into the case, pour the resin. The forward search part of the probe is L1 (Tx), although this does not matter. Then we select the elements marked on the diagram with an asterisk in the phase shifter, so that the detector responds to all metals and does not detect ferrite and stones containing iron compounds, and the waveform at its output is rectangular. 

At this point, it may be necessary to correct the value of the Rx capacitor. The coils do not contain a core, and the fiberglass rod seen in the pictures simply reinforces the mechanical design of the sensor. Point L1 on the assembly drawing is the point of connection of the total mass of all coils. 

The circuit checks amplitude and phase with Rx. Thanks to this, the detector will distinguish between non-ferrous metals, and the pinpointer can also eliminate the influence of the soil.