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The circuit diagram of the metal detector on Arduino with metal discrimination is shown in the following figure.

When metal is detected, the device will emit a sound signal, and the LCD display will display the proximity of the metal using a bar graph, as well as indicate the type of metal – iron (ferrous) or non-ferrous (non-ferrous).

The device is an induction-balanced metal detector operating at a very low frequency (VLF). The metal detector contains a transmitting and receiving inductance coil. As in all circuits of similar detectors, the balance between the coils is very important for our device. The potentiometer in the detector circuit is used to eliminate the influence of the out-of-phase component of the signal – it brings the phase shift to zero, and the in-phase component is zeroed using the appropriate arrangement of the coils – according to the operating principle of IB detectors.

Each induction coil is made by winding 64 turns of 0.5 mm2 enameled copper wire onto a D shape with a diameter of 11 cm. The coil structure is then wrapped with tape and shielded with aluminum foil, after which tinned copper wire is attached to it – it is necessary to leave a small gap in the foil to attach it. After that, both coils are fixed to a plastic base. The appearance of the assembled induction coils for the metal detector is shown in the following figure.

You can watch the assembly process in more detail in the video provided at the end of the article. The appearance of the assembled metal detector structure is shown in the following figure.

To set up the project, we first need to determine the resonant frequency of the oscillatory circuit in our circuit. To do this, you can use the formula known from the physics course, online calculators, or you can measure it with an oscilloscope. If you assembled the coils in the described way, then the resonant frequency of our circuit should be approximately 7.64 kHz. If you received a different resonant frequency value, then you need to make the appropriate changes to the next line of the program:

#define TIMER1_TOP (249) // fine-tune the frequency

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As you can see in the video below, the results of the metal detector were quite impressive. In the absence of metal, the device shows stable operation. A metal circle with a diameter of 15 cm is detected at a distance of more than 30 cm. Larger metal objects are detected at distances exceeding 40-50 cm. A small coin can be detected at a distance of 15 cm (in the air).

To power the metal detector, the author of the project used two lithium batteries, which, when connected in series, provide a supply voltage of 7.4 V – this voltage is supplied to the Vin contact of the Arduino board. The power consumption of the device does not exceed 20 mA, so it will work for quite a long time from such batteries.

To significantly increase the sensitivity of the metal detector, the author of the project proposes to control the transmitting coil using a powerful MOSFET transistor – in the future, he plans to publish the results of such an experiment on the project page.

Note : recommendations for practical assembly of this metal detector project from an active reader of our site named Alexander with a description, photographs and video can be downloaded fromthe following link .

// Induction balance metal detector
// We run the CPU at 16MHz and the ADC clock at 1MHz (АЦП работает на 1 МГц). ADC resolution is reduced to 8 bits at this speed. (разрешение АЦП уменьшено до 8 бит)
// Timer 1 is used to divide the system clock by about 256 to produce a 62.5kHz square wave. (Таймер 1 использовается для формирования прямоугольных импульсов с частотой 62,5 кГц)
// This is used to drive timer 0 and also to trigger ADC conversions.
// Timer 0 is used to divide the output of timer 1 by 8, giving a 7.8125kHz signal for driving the transmit coil.
// Таймер 0 делит выход таймера 1 на 8, и, таким образом, формирует сигнал частотой 7.8125kHz для управления передающей катушкой
// This gives us 16 ADC clock cycles for each ADC conversion (it actually takes 13.5 cycles), and we take 8 samples per cycle of the coil drive voltage.
// The ADC implements four phase-sensitive detectors at 45 degree intervals. Using 4 instead of just 2 allows us to cancel the third harmonic of the
// coil frequency.
// Timer 2 will be used to generate a tone for the earpiece or headset. (таймер 2 используется для генерации тона для наушников)
// Other division ratios for timer 1 are possible, from about 235 upwards.
// Wiring:
// Connect digital pin 4 (alias T0) to digital pin 9
// Connect digital pin 5 through resistor to primary coil and tuning capacitor
// Connect output from receive amplifier to analog pin 0. Output of receive amplifier should be biased to about half of the analog reference.
// When using USB power, change analog reference to the 3.3V pin, because there is too much noise on the +5V rail to get good sensitivity.
#include <LiquidCrystal.h>
#include <LcdBarGraph.h>
#define max_ampAverage 200
LiquidCrystal lcd(6, 7, 10, 11, 12, 13);
LcdBarGraph lbg(&lcd, 16, 0, 1);
#define TIMER1_TOP (259) // can adjust this to fine-tune the frequency to get the coil tuned (see above) (это значение используется для точной настройки частоты катушки)
#define USE_3V3_AREF (1) // set to 1 of running on an Arduino with USB power, 0 for an embedded atmega28p with no 3.3V supply available
// Digital pin definitions
// Digital pin 0 not used, however if we are using the serial port for debugging then it’s serial input
const int debugTxPin = 1; // transmit pin reserved for debugging (передающий контакт, зарезервированный для целей отладки)
const int encoderButtonPin = 2; // encoder button, also IN0 for waking up from sleep mode
const int earpiecePin = 3; // earpiece, aka OCR2B for tone generation
const int T0InputPin = 4;
const int coilDrivePin = 5;
const int LcdRsPin = 6;
const int LcdEnPin = 7;
const int LcdPowerPin = 8; // LCD power and backlight enable
const int T0OutputPin = 9;
const int lcdD4Pin = 10;
const int lcdD5Pin = 11; // pins 11-13 also used for ICSP
const int LcdD6Pin = 12;
const int LcdD7Pin = 13;
// Analog pin definitions (используемые аналоговые контакты)
const int receiverInputPin = 0;
const int encoderAPin = A1;
const int encoderBpin = A2;
// Analog pins 3-5 not used
// Variables used only by the ISR
int16_t bins[4]; // bins used to accumulate ADC readings, one for each of the 4 phases (используются для хранения значений, считываемых с АЦП)
uint16_t numSamples = 0;
const uint16_t numSamplesToAverage = 1024;
// Variables used by the ISR and outside it
volatile int16_t averages[4]; // when we’ve accumulated enough readings in the bins, the ISR copies them to here and starts again
volatile uint32_t ticks = 0; // system tick counter for timekeeping
volatile bool sampleReady = false; // indicates that the averages array has been updated
// Variables used only outside the ISR
int16_t calib[4]; // values (set during calibration) that we subtract from the averages (значения, устанавливаемые во время калибровки, в дальнейшем мы их вычитаем из средних значений)
volatile uint8_t lastctr;
volatile uint16_t misses = 0; // this counts how many times the ISR has been executed too late. Should remain at zero if everything is working properly.
const double halfRoot2 = sqrt(0.5);
const double quarterPi = 3.1415927/4.0;
const double radiansToDegrees = 180.0/3.1415927;
// The ADC sample and hold occurs 2 ADC clocks (= 32 system clocks) after the timer 1 overflow flag is set.
// This introduces a slight phase error, which we adjust for in the calculations.
const float phaseAdjust = (45.0 * 32.0)/(float)(TIMER1_TOP + 1);
float threshold = 5.0; // lower = greater sensitivity. 10 is just about usable with a well-balanced coil.
// The user will be able to adjust this via a pot or rotary encoder. (эту границу можно сделать настраиваемой с помощью потенциометра или энкодера)
void setup()
{
lcd.begin(16, 2);// LCD 16X2
pinMode(encoderButtonPin, INPUT_PULLUP);
digitalWrite(T0OutputPin, LOW);
pinMode(T0OutputPin, OUTPUT); // pulse pin from timer 1 used to feed timer 0
digitalWrite(coilDrivePin, LOW);
pinMode(coilDrivePin, OUTPUT); // timer 0 output, square wave to drive transmit coil
cli();
// Stop timer 0 which was set up by the Arduino core
TCCR0B = 0; // stop the timer
TIMSK0 = 0; // disable interrupt
TIFR0 = 0x07; // clear any pending interrupt
// Set up ADC to trigger and read channel 0 on timer 1 overflow
#if USE_3V3_AREF
ADMUX = (1 << ADLAR); // use AREF pin (connected to 3.3V) as voltage reference, read pin A0, left-adjust result
#else
ADMUX = (1 << REFS0) | (1 << ADLAR); // use Avcc as voltage reference, read pin A0, left-adjust result
#endif
ADCSRB = (1 << ADTS2) | (1 << ADTS1); // auto-trigger ADC on timer/counter 1 overflow
ADCSRA = (1 << ADEN) | (1 << ADSC) | (1 << ADATE) | (1 << ADPS2); // enable adc, enable auto-trigger, prescaler = 16 (1MHz ADC clock)
DIDR0 = 1;
// Set up timer 1.
// Prescaler = 1, phase correct PWM mode, TOP = ICR1A
TCCR1A = (1 << COM1A1) | (1 << WGM11);
TCCR1B = (1 << WGM12) | (1 << WGM13) | (1 << CS10); // CTC mode, prescaler = 1
TCCR1C = 0;
OCR1AH = (TIMER1_TOP/2 >> 8);
OCR1AL = (TIMER1_TOP/2 & 0xFF);
ICR1H = (TIMER1_TOP >> 8);
ICR1L = (TIMER1_TOP & 0xFF);
TCNT1H = 0;
TCNT1L = 0;
TIFR1 = 0x07; // clear any pending interrupt
TIMSK1 = (1 << TOIE1);
// Set up timer 0
// Clock source = T0, fast PWM mode, TOP (OCR0A) = 7, PWM output on OC0B
TCCR0A = (1 << COM0B1) | (1 << WGM01) | (1 << WGM00);
TCCR0B = (1 << CS00) | (1 << CS01) | (1 << CS02) | (1 << WGM02);
OCR0A = 7;
OCR0B = 3;
TCNT0 = 0;
sei();
while (!sampleReady) {} // discard the first sample (отбрасываем первый отсчет)
misses = 0;
sampleReady = false;
Serial.begin(19200);
}
// Timer 0 overflow interrupt (прерывание от таймера 0). This serves 2 purposes: (служит для 2-х целей)
// 1. It clears the timer 0 overflow flag. If we don’t do this, the ADC will not see any more Timer 0 overflows and we will not get any more conversions.
// 2. It increments the tick counter, allowing is to do timekeeping. We get 62500 ticks/second.
// We now read the ADC in the timer interrupt routine instead of having a separate comversion complete interrupt.
ISR(TIMER1_OVF_vect)
{
++ticks;
uint8_t ctr = TCNT0;
int16_t val = (int16_t)(uint16_t)ADCH; // only need to read most significant 8 bits (нам нужно считывать только 8 наиболее значащих бит)
if (ctr != ((lastctr + 1) & 7))
{
++misses;
}
lastctr = ctr;
int16_t *p = &bins[ctr & 3];
if (ctr < 4)
{
*p += (val);
if (*p > 15000) *p = 15000;
}
else
{
*p -= val;
if (*p < –15000) *p = –15000;
}
if (ctr == 7)
{
++numSamples;
if (numSamples == numSamplesToAverage)
{
numSamples = 0;
if (!sampleReady) // if previous sample has been consumed
{
memcpy((void*)averages, bins, sizeof(averages));
sampleReady = true;
}
memset(bins, 0, sizeof(bins));
}
}
}
void loop()
{
while (!sampleReady) {}
uint32_t oldTicks = ticks;
if (digitalRead(encoderButtonPin) == LOW)
{
// Calibrate button pressed. We save the current phase detector outputs and subtract them from future results.
// This lets us use the detector if the coil is slightly off-balance.
// It would be better to everage several samples instead of taking just one.
for (int i = 0; i < 4; ++i)
{
calib[i] = averages[i];
}
sampleReady = false;
Serial.print(“Calibrated: “);
lcd.setCursor(0,0);
lcd.print(“Calibrating… “);
for (int i = 0; i < 4; ++i)
{
Serial.write(‘ ‘);
Serial.print(calib[i]);
lcd.setCursor(0,1);
lcd.print(‘ ‘);
lcd.print(calib[4]);
lcd.print(” “);
}
Serial.println();
}
else
{
for (int i = 0; i < 4; ++i)
{
averages[i] -= calib[i];
}
const double f = 200.0;
// Massage the results to eliminate sensitivity to the 3rd harmonic, and divide by 200
double bin0 = (averages[0] + halfRoot2 * (averages[1] – averages[3]))/f;
double bin1 = (averages[1] + halfRoot2 * (averages[0] + averages[2]))/f;
double bin2 = (averages[2] + halfRoot2 * (averages[1] + averages[3]))/f;
double bin3 = (averages[3] + halfRoot2 * (averages[2] – averages[0]))/f;
sampleReady = false; // we’ve finished reading the averages, so the ISR is free to overwrite them again
double amp1 = sqrt((bin0 * bin0) + (bin2 * bin2));
double amp2 = sqrt((bin1 * bin1) + (bin3 * bin3));
double ampAverage = (amp1 + amp2)/2.0;
// The ADC sample/hold takes place 2 clocks after the timer overflow
double phase1 = atan2(bin0, bin2) * radiansToDegrees + 45.0;
double phase2 = atan2(bin1, bin3) * radiansToDegrees;
if (phase1 > phase2)
{
double temp = phase1;
phase1 = phase2;
phase2 = temp;
}
double phaseAverage = ((phase1 + phase2)/2.0) – phaseAdjust;
if (phase2 – phase1 > 180.0)
{
if (phaseAverage < 0.0)
{
phaseAverage += 180.0;
}
else
{
phaseAverage -= 180.0;
}
}
// For diagnostic purposes, print the individual bin counts and the 2 indepedently-calculated gains and phases
Serial.print(misses);
Serial.write(‘ ‘);
if (bin0 >= 0.0) Serial.write(‘ ‘);
Serial.print(bin0, 2);
Serial.write(‘ ‘);
if (bin1 >= 0.0) Serial.write(‘ ‘);
Serial.print(bin1, 2);
Serial.write(‘ ‘);
if (bin2 >= 0.0) Serial.write(‘ ‘);
Serial.print(bin2, 2);
Serial.write(‘ ‘);
if (bin3 >= 0.0) Serial.write(‘ ‘);
Serial.print(bin3, 2);
Serial.print(” “);
Serial.print(amp1, 2);
Serial.write(‘ ‘);
Serial.print(amp2, 2);
Serial.write(‘ ‘);
if (phase1 >= 0.0) Serial.write(‘ ‘);
Serial.print(phase1, 2);
Serial.write(‘ ‘);
if (phase2 >= 0.0) Serial.write(‘ ‘);
Serial.print(phase2, 2);
Serial.print(” “);
// Print the final amplitude and phase, which we use to decide what (if anything) we have found)
if (ampAverage >= 0.0) Serial.write(‘ ‘);
Serial.print(ampAverage, 1);
Serial.write(‘ ‘);
lcd.setCursor(0,0);
lcd.print(” “);
lcd.print(ampAverage);
lcd.setCursor(0,1);
lbg.drawValue(ampAverage, max_ampAverage);
if (phaseAverage >= 0.0) Serial.write(‘ ‘);
Serial.print((int)phaseAverage);
// Decide what we have found and tell the user
if (ampAverage >= threshold)
{
// When held in line with the centre of the coil:
// – non-ferrous metals give a negative phase shift, e.g. -90deg for thick copper or aluminium, a copper olive, -30deg for thin alumimium.
// Ferrous metals give zero phase shift or a small positive phase shift.
// So we’ll say that anything with a phase shift below -20deg is non-ferrous.
if (phaseAverage < –20.0)
{
Serial.print(” Non-ferrous”);
lcd.setCursor(0,0);
lcd.print(“NonFerous “);
}
else
{
Serial.print(” Ferrous”);
lcd.setCursor(0,0);
lcd.print(“Ferrous “);
}
float temp = ampAverage;
int thisPitch = map (temp, 10, 200, 100, 1500);
tone(3, thisPitch,120);
while (temp > threshold)
{
Serial.write(‘!’);
temp -= (threshold/2);
}
}
Serial.println();
}
while (ticks – oldTicks < 8000)
{
}
}

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