  The Open Lock-In Amplifier (OLIA) is a microcontroller-based digital lock-in
amplifier built from a small number of inexpensive and easily sourced
electronic components. Despite its small credit card-sized form-factor and low
build-cost of around US$35, OLIA is a capable instrument that offers many
features associated with far costlier commercial devices. Key features include
dual-phase lock-in detection at multiple harmonic frequencies up to 50 kHz,
internal and external reference modes, adjustable levels of input gain, a
choice between low-pass filtering and synchronous filtering, noise estimation,
and a comprehensive programming interface for remote software control. OLIA
comes with an optional optical breakout board that allows noise-tolerant
optical detection down to the 40 pW level. OLIA and its breakout board are
released here as open hardware, with technical diagrams, full parts-lists, and
source-code for the firmware provided as Supporting Information.
