Introduction

Author: Teodor-Emilian Petre, Microchip Technology Inc.

This application note shows how a proximity metal detector can be designed using the integrated Operational Amplifier (OPA) of the PIC18-Q71 device family.

This application focuses on an inductive sensor consisting of one capacitor and one inductive sensor placed in parallel. The Pulse-Width Modulation (PWM) configures and describes its period as equal to the half period of the resonant signal. This signal is represented by period, amplitude and damping characteristics. This PWM signal is used to charge the capacitor. The resulting signal of the LC metal detector oscillates for a short period in the form of a damped oscillation.

The capacitor and inductor values are chosen using the parallel LC equation that indicates the LC parallel circuit oscillation frequency. After selecting the desired oscillation frequency, the inductor or the capacitor values can be calculated. The environment conditions can influence the oscillation frequency, with the possibility of noticing minor variations due to humidity, temperature or an electric or magnetic field around the sensor circuit. Bringing a metal object close to the coil changes the oscillating signal, decreasing the signal period, signal amplitude or the signal damping period.

Required components for building this project:

When the resonance conditions are met and when the inductive reactance is equal to the capacitive reactance, the reactive branch current is identical and of reverse way. They cancel each other to give minimum current in the circuit. The resonant frequency is deducted using the following formula:

f = 1 2 ⁢ ⁢ * π * L ⁢ *   C ⁢