Small inductance changes on the order of 0.01% can easily be sensed over a wide range with a simple circuit connected to a serial port (see the figure). Using a classic Colpitts oscillator interfaced to an eight-pin Atmel ATtiny12-8 microprocessor, the period of multiple oscillator cycles—which is proportional to the inductance—can be sensed. Applications include metallic proximity sensing and seismic detection.
The Colpitts oscillator output is level-shifted using R4 and R5 to a zero of about 1.2 V, which is the microprocessor's analog comparator reference voltage. The oscillation frequency of a Colpitts oscillator is:
F = 1/(2π√LC
where C = (C1 × C2)/(C1 + C2), the equivalent capacitance across the inductance.
The sensed inductance is:
L = 1/(((2πN)/T)2)C
where N is the number of oscillator cycles for the period T, and T is the time of the frequency measurement.
The RISC-based microprocessor (operating at an instruction rate of 8 MIPS) counts the number of LC oscillator cycles between a fixed number of crystal-oscillator clocks. With the circuit shown, values of C1 and C2 should be chosen so that the maximum LC oscillator frequency is about 500 kHz.
Frequency measurements are made for 32.5 ms. At the end of each measurement, four hex ASCII characters representing the number of cycles of the LC oscillator are serially transmitted. The online version of this article at www.elecdesign.com includes the ATtiny12 source code and HEX file.
