PIRATE metal detector - self-assembly kit

The kit is intended for the manufacture of a metal detector that allows you to detect objects made of ferrous and non-ferrous metals.
The kit includes a set of parts, a board and a description of the assembly (the speaker is not included in the kit).
Specifications:
Coin detection depth 20 - 25 cm
Depth of detection of large objects 100 - 150 cm
Supply voltage, 9 - 14 V
Current consumption, 60 - 70 mA
Impulse current consumption, 1 - 2 A
Board dimensions, 45 x 52 mm

Circuit diagram

Layout of elements

General description:

The device consists of two main nodes: transmitting and receiving. The transmitting node consists of a pulse generator on the NE555 chip and a powerful key on the IRF740 transistor. The receiving unit is assembled on a TL072 chip and a BC547 transistor. The excitation signal is transmitted periodically to the sensor coil, in the form of pulses with a duration of 125-150 μs and a frequency of 125-150 Hz. In conducting objects, damped eddy currents are induced, which excite a damped electromagnetic field. Depending on the conductive properties and the size of the object, the signal changes its shape and duration.

Search coil manufacturing:

For the manufacture of a search coil, you will need a rigid non-metallic mandrel Ø 200-250 mm, a copper winding enameled wire PEV (Ø0.4 - 0.6 mm) 20 meters long, a copper stranded insulated wire 2x1.5 mm2 120 - 150 cm long to connect the coil with payment. Wrap 25 turns of winding wire tightly onto the mandrel. Fix the ends of the wire and connect by soldering with a stranded insulated wire to the board to the contacts L1.

Assembling and setting up a metal detector

Solder an 8 ohm 0.5 W speaker to the SP1 contacts. The contacts P1 are powered by 9-14 V (preferably from a 12 V 1.2 Ah battery). After turning on the device, by rotating the tuning resistor R8, find a position where the clicks in the speaker will subside as much as possible, and the noise will not start to grow, after which, using the variable resistor R9, perform the same operation, but more accurately, the maximum sensitivity of the metal detector is at the threshold of sound. The sensitivity of a metal detector can be improved by changing the number of coil turns. With a mandrel diameter of 250 mm, and a winding wire of Ø0.5 mm, this is approximately 14 turns. After tuning is complete, the coil can be hardened by wrapping it with tape, electrical tape, or impregnating it with epoxy.

It is recommended to varnish the soldered and debugged board. This will prevent the parameters from drifting when the environmental conditions change and ensure stable and reliable operation of the device.

The set includes:

U1 NE555
U2 TL072
VT1 S9015
VT2 BC547
VT3 IRF740
D1 1N4148
D2 1N4148
R1 120 K
R2 47 K
R3 68 K
R4 68 K
R5 100 K
R6 1 M
R7 1.6K
R8 (trim) 50 K
R9 (variable) 20 K
R10 910 - 1 K
R11 470 - 510
R12 33 - 100
R13 150
R14 430 - 470 K
R15 11
R16 360, 2W
R17 360, 2W
C1 2200x16 V
C2 0.1 uF
SZ 0.1 uF
C4 10x25 V
C5 10x25 V
C6 1000 pF
C7 1x50 V
C8 1x50V
Printed circuit board
Variable Resistor Knob
Schematic diagram