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Simple metal detector

Simple and reliable metal detector

The sensitivity of this detector:

This metal detector discovers a can at 20 cm, a cell phone up to 15 cm, a “crown” battery up to 10 cm, 5 rubles a coin up to 8 cm .

At this distance, the tone of the generator in the headphones barely changes; at a closer distance, the tone increases. This metal detector discriminates the non-ferrous  and ferromagnetic metals.

Schematic diagram

Simple metal detector

List of components

  1. Chip CD4011 ;
  2. Transistor – low-power low-frequency, for example BC546, BC945, 2SC639, 2SC1815 ;
  3. Diode – any low-power, for example in4148, in4001,
  4. Variable resistor – 3 pcs (1 kΩ, 5 kΩ, 20 kΩ with switch or switch separately);
  5. Permanent resistor – 5 pcs (22 Ohms, 4.7 kOhm, 1.0 kOhm, 10 kOhm, 470 kOhm);
  6. Ceramic, and even better mica capacitors – 5 pcs: 1000 pF -3pcs, 22 nF -2pcs, 300 pF);
  7. Electrolytic capacitor (100.0 μF x 16V) – 1 pc;
  8. The wire PEL, PEV, PETV, etc., with a diameter of 0.4-0.7 mm;
  9. Low impedance headphones (from the player);
  10. Battery 9V.

Simple metal detectorPrinted circuit board PCB

Simple metal detector

Simple metal detector

Attention! To exclude interference and the influence of human hands when touching the regulators – the case of variable resistors must be connected to the ground of the board.

With a properly soldered metal detector circuit, serviceability and the correct value of the elements, a correctly made search coil, the device works without problems. If the first time you turn on the headphones you cannot hear the beeping and frequency changes when adjusting the “FREQUENCY” knob, then you need to select a resistor (10 kOhm) , which is in series with the regulator  and/or capacitor in this generator (300 pF).

When the generator is excited, whistling, hissing, distortion appear, solder a 1000 pF capacitor (102) with pin 6.

Simple metal detectorUse an oscilloscope or frequency meter to look at the frequencies of the signals at pins 5 and 6 of CD4011BE . To achieve their equality by the above-described tuning method. The operating frequency of the generators themselves can range from 80 to 200 kHz.

A protective diode (any low-power one) is needed to exclude damage to the microcircuit when the power battery is turned on incorrectly (which often happens :).

Building the search coil

Turns are wound on a mandrel with a diameter of 15-25 cm (for example, a bucket or a shuttle made of thick wire or plywood – the smaller the diameter, the lower the sensitivity, but the greater the selectivity of small metals). Choose for what purpose you need it.

Simple metal detector

A wire is used in the varnish insulation of PEL, PEV, PETV …, with a diameter of 0.4 – 0.7 mm (well suited in old color TVs from a demagnetization coil) and contains about 100 turns (you can wrap from 80 to 120 turns ) Wrap tightly over the wire with electrical tape.

Simple metal detector

Then we wrap the coil over the electrical tape with a strip of foil, leaving a 2-3 cm section not wrapped. The foil can be taken from some types of cables or, in extreme cases, cut into chocolate strips of 2 cm wide.

Next, over the foil we wrap a non-insulated wire (better tinned). The beginning of the wire remains on the coil, and the end is soldered to the body, i.e. minus the power of the board.

Photo of the finished coil. It remains to wrap electrical tape on top.

Simple metal detector

We connect the coil with the circuit board with a double shielded wire (screen to the housing). The wire can be taken from old  tape recorder or a bass cable (audio-video) for connecting a TV to a DVD, etc.

Correct operation of the metal detector:  

When the frequency control is turned on, we set a low-frequency rumble in the headphones, when approaching the metal the frequency changes.

You can use the second option so that the rumble in your ears doesn’t stand, set zero beats, i.e. combine two frequencies. Then there will be silence in the headphones, but as soon as we bring the coil to the metal, the frequency of the search generator changes and a squeak appears in the headphones. When the coil is closer to the metal, the frequency is higher  in the headphones. But the sensitivity with this method is not great. The device will only react when the generators are very detuned, for example.

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