При помощи контроллера Arduino можно собрать простейший детектор металла.
Кроме контроллера, для этого понадобится всего несколько деталей: катушка из проволоки (60 витков провода 0.4 мм, диаметр — 6.3 см), резистор (100 Ом) и конденсатор (10 мкФ). Для звуковой индикации используется обычный бузер.

Используя Arduino Nano и 9-вольтовую батарейку (типа Крона) — можно уместить всё устройство в коробочку из-под тик-така.

Код для Arduino — metal_detector.ino

//Metal detector based on pulse delay in LR circuit
//Connect a coil beween pin 10 and pin 8,
//a 100 Ohm resistor between pin 8 and ground,
//a speaker in series with a 10muF capacitor between pin 12 and ground,
//a reset-button between A0 and ground.
//Reference coil: 60 turns, diameter 6cm, AWG26 (0.4mm) wire. L~300muH, R~2Ohm
//
//R. Oldeman Oct 11 2020, CC BY-NC-SA 4.0 licence

#define debug false

//some parameters
#define sensitivity  5000 //threshold as a fraction of reference
#define aimtime    160000 //aim for measurement every 160k cycles (=10ms)
#define LEDpulselen 16000 //length of the LED pulse in clock cycles
#define cycles         40 //number of clock cycles reserved for calculations
#define printfrac      50 //fraction of measurements printed when in debug mode

//pin definitions - cannot change freely!
#define pulsepin 10 //must be pin 10 - TIMER1B output
#define probepin  8 //must be pin 8  - input capture pin
#define LEDpin   13 //if changed also need to update the direct-port writing
#define soundpin 12 //if changed also need to update the direct-port writing
#define resetpin A0 //can be changed freely

//global variables - shared between main loop and measurement function
long int imeas=0;  //counts the number of measurements performed
int absdiff;       //absolute value of difference wrt reference
long int phase=0;  //tracks integral of absdiff
long int phasemax; //value at which the phase counter turns over
int LEDcycles=0;   //number of cycles that the LED pin stays high
int LEDcycle=0;    //cycle index of the LED pulse

void setup() {
  noInterrupts(); //disable all inerrupts to avoid glitches

  if(debug)Serial.begin(9600);

  pinMode(pulsepin,OUTPUT);
  digitalWrite(pulsepin,LOW);
  pinMode(probepin,INPUT);
  pinMode(LEDpin,OUTPUT);
  digitalWrite(LEDpin,LOW);
  pinMode(soundpin,OUTPUT);
  digitalWrite(soundpin,LOW);
  pinMode(resetpin,INPUT_PULLUP);

  //setup timer1 to FASTPWM on pin 10
  TCCR1A=0B00100011;
  TCCR1B=0B00011001;
}

//perform measurement of delay of the trailing edge
long int meas(int period, int minlen, int maxlen, int steplen, int nloop){
  long int sum=0;
  int nmiss=0;
  OCR1A=period;        //set the period
  TIFR1 |= 0B00000001; //clear the timer overflow bit
  for(int iloop=0; iloopphasemax){
        phase-=phasemax;
        LEDcycle=LEDcycles;
        PORTB=(PORTB|0B00100000); //switch on LED
        PORTB=(PORTB^0B00010000); //invert sound pin - click!
      }
      //switch off LED when it's been on long enough
      if(LEDcycle>0){
        LEDcycle--;
        if(LEDcycle==0)PORTB=(PORTB&(!0B00100000));
      }
    }
  }
  if (nmiss>0)sum=0; //invalidate the result if there have been missing pulses
  return sum;
}

//flash LED with error code indefinitely if there's been an error
void go_error(byte errorcode){
  if(debug){
    Serial.print("ERROR ");
    Serial.print(errorcode);
    if(errorcode==1)Serial.println(": No pulse on probe pin - check connections");
    if(errorcode==2)Serial.println(": Delay too short - try more windings");
    if(errorcode==3)Serial.println(": Delay too long - try fewer windings");
    if(errorcode==4)Serial.println(": No robust pulse on pulselength scan - check coil resistance");
  }
  //enable in interrupts to use delay() and flash to indicate error code
  interrupts();
  while(true){
    for(byte i=0; i800)go_error(3);           // too long

  //choose pulsing parmeters based on measured delay time
  int minlen=3*tau;
  int maxlen=5*tau;
  int period=maxlen+2*tau+cycles;
  LEDcycles=LEDpulselen/period+1;

  // repeat loop or speed up loop to approach aimed duration of measurement
  int steplen=1;
  int nloop=1;
  float tottime=float(period)*(maxlen-minlen);
  if (tottime>aimtime*(3.0/2.0)) steplen=round(tottime/aimtime);
  if (tottime0)ref+=1;      //absorb slow drifts
      if(diff<0)ref-=1;
    }

    if(debug){
      sum+=diff;
      sumsq+=long(diff)*long(diff);
      if (imeas%printfrac==0){
        float mean=float(sum)/printfrac;
        float rms=sqrt( (float(sumsq)/printfrac) - pow(mean,2.0) );
        Serial.print(imeas);
        Serial.print(" val ref diff phase ");
        Serial.print(val);
        Serial.print(" ");
        Serial.print(ref);
        Serial.print(" ");
        Serial.print(diff);
        Serial.print(" ");
        Serial.print(mean);
        Serial.print(" ");
        Serial.print(rms);
        Serial.println("");
        sum=0; sumsq=0;
      }
    }
    imeas++;
  }
}

Ссылки
Minimal Arduino Metal Detector

По теме
Металлоискатель на Arduino

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