Metal detectors
Theory and practice
A metal detector can be useful around the house to help locate lost coins, jewelry, keys and gas lines. To the hobbyist a metal detector provides relaxation, excitement, the thrill of discovery and why not - profit.
Here are the three main detector types used today:
- 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. The VLF detector combines two 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.
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. They are able to detect objects buried deep underground, more sensitive than VLF detectors.
The Beat-frequency oscillator (BFO) is the simplest type of metal detector 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 of the two input signals.
The difference in frequency is feed to a low-pass filter removing the harmonics. As long as the two oscillators are exactly the same frequency, the output will have no difference signal. If the frequency of the search oscillator shifts slightly, then a frequency difference signal will appear. The frequencies of the two oscillators are normally chosen such way that a shift in the search oscillator frequency will produce a signal in the audio frequency range.
Simple BFO metal detector schematic
This simple metal detector requires only a handful of components and an evening's work. Built around a cmos4011 IC, is very robust and versatile. The 250 kHz reference oscillator is built with two gates (U1/1 and U1/2) C1, R1 and P2. The search oscillator uses only one gate (U1/3) two capacitors and the search coil. The output of the two oscilators are fed to the fourth gate acting as a mixer, filtered with C4 then amplified with U2.
After assembly adjust the volume control (P2) for comfortable volume and 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 select different values for C1.
- U1: CD4011
- U2: LM349
- U3: 78L05
- R1: 2.2k 5%
- P2: 4.7k lin.
- R3: 330k 5%
- R4: 270k 5%
- R5: 1k 5%
- C1: 390pF (NPO)
- C2,C3,C4: 10nF
- C5: 10uF 16v electrolytic
- C6,C8: 220 uF 16v electrolytic
- C7: 100uf 16v electrolytic
- C9: 100nF ceramic
- P1: 4.7k log
- L1: 22cm in diameter with 14 turns AWG 26
- K1: SPDT toggle switch
- J1= Headphone jack 1/4 or 1/8 inch
- Other parts: 9v battery connector, speaker or headphones