Microcontroller-based digital lock-in kelvin probe system using a self-resonant piezoelectric buzzer plate

The vibrating parallel-plate capacitor-based Kelvin probe (KP) technique is a well-established method for measuring the contact potential difference (CPD) between a sample and a reference material, widely used in surface science and device characterization. In the present work, a compact and low-cost KP system is developed using simple and easily accessible electronic components for ambient-condition CPD measurements. A 35 mm diameter piezoelectric buzzer plate is employed for periodic modulation of a metallic reference probe, providing a compact and mechanically rigid alternative to conventional solenoid-based modulation systems. The vibration-induced capacitive current is measured using a homemade transimpedance amplifier and processed using a microcontroller-based digital lock-in amplifier (D-LIA) implemented on a Teensy 4.1 platform, enabling amplitude and phase extraction through numerical mixing and low-pass filtering techniques. The piezoelectric modulation scheme further enables the implementation of a compact self-resonant driver circuit operating near 1.7 kHz, eliminating the need for an external function generator during normal operation. The developed system was experimentally validated through analog-to-digital calibration, capacitance measurements, phase-sensitive RC circuit analysis, and KP measurements on copper-aluminum systems under ambient oxidation conditions, demonstrating a CPD resolution of ∼10 mV (standard deviation) when operated with an external function generator and ∼100 mV using the self-resonant driver. The modular architecture of the system also provides the possibility for future implementation in scanning-based surface potential mapping, scanning tunneling microscopy tip-approach protocols, low-frequency spectroscopy measurements, and other phase-sensitive low-signal detection applications. Detailed electronic circuits, digital signal processing methods, and hardware implementations are presented.