Metal detectors

Metal detectors

Theory and practice

Metal detectors are used in a wide range of applications from landmine detection to safety in airports, office buildings or schools. They can also be useful around the house to help locate lost coins, jewelry, keys and gas lines.
Metal detectors helped archaeologists in the discovey of precious artifacts and coins that were once the everyday items of use by our ancestors. Until recently, this privilege was reserved for the lucky few who could afford the expensive instrument. But with the advances in electronics and technology the price of these machines dropped to an affordable level.
Today, inexpensive, high quality consumer-oriented metal detectors provide millions of hobbyists around the world with an opportunity to discover hidden treasures, providing relaxation, excitement, the thrill of discovery and why not - profit.
As you start your search for the perfect metal detector, you will quickly discover that there are a multitude of detectors from which to choose from. There are machines using different technologies such as BFO (Beat Frequency Oscillator), Off-Resonance, IB (Induction Balance), VLF (Very Low Frequency), VLF/TR, TR (Transmit-Receive), PI (Pulse Induction), or RF (Radio-Frequency or Two-box detectors). Innovations in the field of metal detecting are a still going on -- there are new patents and original designs born every day.
We will cover only the three main detector types you will likely to encounter in your quest for the perfect metal detector for treasure hunting and exploration:

• VLF or Very Low Frequency
• PI or Pulse Induction
• BFO or Beat-frequency oscillator

Very Low Frequency (VLF) detectors are the most versatile metal detector types, based on the range of metallic objects you can find with them. They are IB (Induction Balance) detectors using very low frequencies. As all IB designs, the VLF detector combines two balanced coils: the outer coil acts as a transmitter, using alternating current to create a magnetic field that is distorted by a metallic object and the inner coil acting as a receiver, reading the secondary magnetic field created by the conductive object. This magnetic field is amplified and converted to an audio tone. The phase demodulators help to discriminate among types of objects.
Example VLF design: Heathkit Groundtrack GR-1290 VLF metal detector

Pulse Induction (PI) metal detectors are sending repeated pulses of electrical current to the search coil, producing a magnetic field. The coil transmits a pulse toward the ground, generating an answering pulse from the target object. A sampling circuit measures the pulse and sends it to an integrator, which generates an audio tone.
PI outperforms the VLF/TR in areas where little trash is located, on saltwater beaches or mineralized ground, since they are capable of ignoring both conductive salts and mineralization simultaneously.
Pulse Induction detectors are able to detect objects buried deep underground, but they are sensitive to iron and do not have the ability to discriminate against different types of metals. This flaw makes their use on inland sites extremely difficult.
Example PI circuit: White's Surfmaster PI schematic diagram

The Beat-frequency oscillator (BFO) is the simplest (and oldest) type of metal detector technology and is a good starting point for learning how metal detectors work. The basic beat-frequency metal detector employs two radio frequency oscillators which are tuned near the same frequency. The first is called the search oscillator and the other is called the reference oscillator.
The outputs of the two oscillators are fed into a mixer which produces a signal that contains the sum and difference frequency components. This signal is feed to a low-pass filter removing the harmonics. As long as the two oscillators are tuned to the same frequency, the output will have no signal.
When a metallic object disturbs the magnetic field of the search coil, the frequency of the search oscillator shifts slightly and the detector will produce a signal in the audio frequency range.
Although once popular, BFO's are no longer being made by professional metal detector manufacturers. They are simple and inexpensive, but do not offer the accuracy and control of modern PI or VLF detectors. Attempts have been made to add new features such as discrimination and more advanced models were produced in the 1970s, but they were soon replaced by recent, more sophisticated technology.
BFO designs are still used in cheap hand-held devices and in low quality, toy type detectors. The vintage BFO detector is more of a curiosity and collector's item than a usable piece of equipment.

Example BFO schematic: Simple BFO metal detector circuit

Simple BFO Metal Detector Schematic Diagram

This simple BFO metal detector requires only a few of components and an evening's work. The two oscillators are simple Colpitts designs using BJT transistors. The reference oscillator's frequency is approximately 370kHz, slightly tunable with the help of a silicon varactor diode. The outputs of the two oscillators are fed to a mixer made with Q3 and Q4. The signal then goes through a low-pass filter (R13, C13) and a JFET preamp. The LM386 audio amplifier has a gain of 20, more than enough for most headphones. If you need more gain, you can add a 10uF capacitor between pins 1 and 8.

Fig. 1: Simple BFO metal detector schematic diagram

Parts list:

Resistors: R1, R3, R5, R7: 22kΩ resistors R2, R6: 1kΩ resistors R4, R9, R12: 15kΩ resistors R8, R10, R11: 47kΩ resistors R13: 2.2kΩ resistor R14: 1MΩ resistor R15: 8.2kΩ resistor R16: 680Ω resistor R17: 10Ω resistor P1: 10kΩ lin. potentiometer (Tune) P2: 10kΩ log. potentiometer (Volume) Other parts: L1: 10cm (4in.) diameter, 20 turns, AWG 22 L1: 82uH inductor SW1: SPDT toggle switch J1: Headphone jack 1/4 or 1/8 inch

Capacitors: C1, C6, C7, C12, C14: 100nF capacitors C2, C8: 22nF low temp. coef. capacitors C3, C9: 2.2nF low temp. coef. capacitors C4, C10: 10pF ceramic capacitors C5, C11: 4.7uF/16V electrolytic C13: 10nF capacitor C15: 47nF capacitor C16, C17: 220uF/16V electrolytic Active components: D1: NTE618 silicon varactor diode (20-440pF) Q1-Q4: 2N2222 NPN silicon transistors Q5: 2N5951 JFET transistor IC1: LM386 (Audio amplifier IC)

The coil should have 20 turns of 0.65 mm (AWG 22) enameled copper wire wound on a 10 cm (4in.) diameter form. Wrap the completed loop with strips of aluminum foil or copper shield tape, then connect the shield to the ground. Make sure the Faraday shield has a gap at one point, so it does not make a shorted loop. You should mechanically secure the coil to a nonmetallic form to prevent microphonics.
After assembly, connect the headphones and slowly turn P1. The pitch will get lower until it disappears. Continuing to rotate P1 in the same direction will cause the pitch to rise again. The point at witch the pitch is the lowest and disappears is called "zero beat". If you can not get this zero beat frequency for the entire turn of P1 you may have to increase or decrease the value of L2.
Turn P1 close to the zero beat position (a tone of 50Hz-200Hz), then move the search coil near a metallic object. The tone should change, depending on the size and distance of the metal.
Note: this simple circuit will only detect relative large metallic objects at a short distance. Coins and other small objects will be much harder to find! If you want to build a detector with a performance comparable to commercial products, try a PI or VLF design.

More metal detector schematics:
White's Surfmaster PI metal detector schematic diagram
Heathkit Cointrack Gd-1190 Metal Locator
Heathkit Groundtrack GR-1290 VLF metal detector
White's Classic I metal detector schematic

White's Surfmaster PI schematic diagram

White's Surfmaster PI is a good quality, fully waterproof, lightweight metal detector with an exceptional depth in saltwater or mineralized ground. It's ideal for those wishing to search beaches, in the surf or shallow diving. It works inland too, but such use is not advised as -- like all PI (Pulse Induction) designs -- there is no effective discrimination of ferrous objects. Runs on eight AA size batteries.

Fig. 1: White's Surfmaster PI metal detector schematic diagram - (2000x935 PNG)

Parts list:

Resistors: R1, R10, R22, R35: 100kΩ R2: 3.3kΩ R3, R7, R8, R13, R14, R21, R33: 1kΩ R4, R5: 100Ω R6: 220Ω 1/2 W R9: 1MΩ R11, R20, R23, R32: 22kΩ R12, R27: 10kΩ R15, R16, R17: 220kΩ R18: 47kΩ R26, R29: 51kΩ R28: 210kΩ R34: 150Ω R25: 50kΩ trim potentiometer R10: 100kΩ trim potentiometer R33: 1kΩ trim pot. (Volume control) P1: 10kΩ potentiometer (THRESH) P2: 100kΩ potentiometer (DISC) Coil: Note: It's best to use the original Surfmaster coil for optimal performance, but you can experiment with different turns, wire gauge and coil diameters, shielding... A reasonable depth and sensitivity can be obtained with a 23cm diameter coil, 20 turns made with 0.5mm (24 gauge) standard copper enamel coated wire.

Capacitors: C1: 22nF C2: 1000uF/25V electrolytic C3, C4, C8, C16: 100nF C5, C6, C7: 470nF C9, C10, C11, C12: 1nF C13, C14, C15: 220uF/10V electrolytic Transistors: Q1: 2N3906 (PNP silicon transistor) Q2: TIP32C (PNP silicon power transistor) Q6: MPSA13 (NPN Darlington transistor) Diodes: D1, D2, D3: 1N4148 (Switching diodes) D6: 1N4001 (Rectifier diode) D7: 1N5819 (Schottky Barrier Rectifier) D5: 1N755 (7.5V Zener diode) Integrated circuits: U1: TLC555 (LinCMOS timer IC) U2: NE5534 (Single Low Noise Op Amp) U3: LM358 (Low Power Dual Op Amp) U4: MC14093B (Quad Schmitt trigger) U5: 78L05 (5V Positive Voltage Regulator) U6: ICL7660 (CMOS switched-capacitor voltage converter) U7: CD4066 (Quad Bilateral Switch)

Notes

• All resistors 1/4 watt +/-5% unless noted otherwise
• Capacitors with values greater than 1uF are aluminium electrolytics (+80%, -20%)
• Capacitors with values less than 1uF are polyester film type (+/-10%)
• Nominal capacitor range of 0.001-0.39uF = polyester film.
• Nominal capacitor range of 0.047-1uF = stacked polyester film.
• ST = Stacked polyester (Panasonic "V" series) +/-5%.
• Simple mod: If TIP32C (Q2) gets excessively hot, use an IRF9640 MOSFET instead. Experiment with different values for R6 to find the optimal value (I use a 390 ohm resistor). Temporarily substitute R6 for a trim potentiometer (470Ω or 1kΩ) to find the optimal dumping resistor for your coil.

Fig. 2: White's Surfmaster PI metal detector

More metal detector schematics:
Heathkit Cointrack Gd-1190 Metal Locator
Heathkit Groundtrack GR-1290 VLF metal detector
White's Classic I metal detector schematic
Simple BFO metal detector schematic diagram

Heathkit Groundtrack GR-1290 VLF metal detector

Fig. 1: Heathkit GR-1290 VLF metal detector schematic diagram

 

Parts list

ICs:

• U1: CA3130S (BiMOS Operational Amplifier)
• U2, U3: LM308 (Precision operational amplifiers)
• U4: LM2901 (Quad Single Supply Comparator IC)

Transistors:

• Q1, Q4, Q9, Q10, Q11, Q12: MPSA20 (NPN 40V 100mA)
• Q2, Q7, Q8, Q13: MPSA55 (PNP 60V 500mA)
• Q3, Q5: 2N4117 (N-channel JFET transistors)
• Q6: MPSA13 (NPN Darlington transistor)

Diodes:

• D1, D3, D4: 1N4149 or 1N4148 (switching diodes)
• D2: 1N5230B (4.7V, 0.5W Zener Diode)

Notes

All resistors are 1/4 watt, 5% tolerance and all capacitors are in microfarads (uF) unless marked otherwise.
Voltages were taken with a 9 Volt supply and a setting with no sound coming from the speaker. The controls were set as follows:

• Volume -- fully counterclockwise
• Pushbutton tune -- center of rotation
• Discriminate -- center of rotation
• Audio Tune -- center of rotation

The original coil that came with the Heathkit project was preassembled at the factory. It had a diameter of around 15cm (6 inch). A larger (30cm, 12inch) coil can be made using the following data:
L1 = 300mm diameter, 90 turns, .4mm wire
L2 = 140mm diameter, 90 turns, .4mm wire
L3 = 130mm diameter, 550 turns, .1mm wire

See also:

Heathkit Cointrack Gd-1190 Metal Locator
White's Classic I metal detector schematic
White's Surfmaster PI metal detector schematic diagram