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Creating a Functional Metal Detector

Apr 26, 2025

Lecture Notes: Building a Metal Detector

Introduction

  • Objective: Building a metal detector to find metals like silver and gold.
  • Common misconceptions and ineffective methods of metal detection.
    • Example: Using a nail or screw in wood with a light bulb.
  • Correct method involves using magnetic fields and inductors.

Correct Metal Detection Method

  • Use of inductors to generate magnetic fields.
  • Primary coil example from a microwave oven transformer.
    • No secondary coil to avoid high voltages.
  • Magnetic fields can detect metal by affecting the inductor.
  • Alternating Current (AC) creates changing magnetic fields.
  • Eddy currents are generated in metal due to changing magnetic fields.
    • Ferromagnetic materials (e.g., iron) become stronger magnets.
    • Non-ferromagnetic materials (e.g., aluminum) create opposing fields.

Key Concepts

  • Ferromagnetic vs Non-ferromagnetic
    • Ferromagnetic (iron-like) materials become magnets.
    • Non-ferromagnetic materials (proposed term: "allium-magnetic") do not.
  • Understanding the behavior of different metals in magnetic fields.

Experimentation

  • Using an LCR meter to measure inductance changes.
  • Observations on how different metals affect inductance.

Oscillator Circuit for Metal Detection

  • Creating an oscillator circuit with a coil and capacitor.
  • Audible tone changes when metal is near the coil.
  • Challenges with this method: limited range, small frequency changes.

Advanced Metal Detector Design

  • Reference to a book by Carl Moorland on metal detectors.
  • Experiments with coils and different frequencies.
  • Observations on how different frequencies affect metal detection.
    • 25 kHz can differentiate between ferromagnetic and non-ferromagnetic metals.

Building the Metal Detector Circuit

  • Design of a ZVS oscillator with a current limiter circuit.
  • Challenges: power dissipation in resistors.
  • Coil design: Using 26 gauge wire and a 10 cm diameter.

Signal Processing

  • Using multiplication of sine waves to detect small frequency changes.
  • Use of a low pass filter to isolate audible frequencies.
  • Circuit design for multiplying and filtering signals.
  • Demonstration of signal changes when metal is near the coil.

Tuning and Calibration

  • Manual tuning of the circuit frequency to detect metal.
  • Detection range varies with the size of the metal.
  • Differentiation between ferromagnetic and non-ferromagnetic metals.
  • Stability and drift issues in oscillator frequency.

Conclusion

  • Successful creation of a metal detector that distinguishes between metal types.
  • Mention of sponsor Keysight's tools and upcoming events.

Additional Information

  • Keysight World Innovate event covering future technology topics such as 5G/6G networks, AI in vehicles, and digital healthcare.

These notes provide a high-level overview of the process of building a metal detector, capturing the main ideas and detailed steps taken during the lecture.

Full transcript