Radio constructor Pulse Metal Detector K158

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Pulse metal detector - a set designed to make a metal detector that allows you to detect objects made of ferrous and non-ferrous metals.

Technical specifications:

Depth of discovery of the coin 5 kopecks, times of the USSR: 18 - 20 cm
Depth of detection of large details: 100 - 150 cm
Supply voltage: 9 - 14 V
Current consumption, 60 .60 - 70 mA
Pulse current consumption: 1 - 2 A
Overall dimensions of the board: 45 X 52 mm

Device description 

  The device consists of two nodes: transmitting and receiving. The transmitting node consists of a pulse generator on the NE555 chip and a powerful key on the BC547. 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.

Schematic diagram, circuit diagram 

Layout of elements

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. The assembly of the amplifier is carried out according to the instructions that come with the designer. The seats on the board and the ratings of the parts are indicated in the instructions.   

Assembly and setup of the metal detector

  Solder an 8 ohm 0.5 W speaker to the SP1 contacts. The contacts P1 are powered by 9-14V (preferably from a 12 V 1.2 A / hour 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. The assembly of the amplifier is carried out according to the instructions that come with the designer. The seats on the board and the ratings of the parts are indicated in the instructions.