  Phase-sensitive detection (PSD) is an important experimental technique that
allows signals to be extracted from noisy data. PSD is also used in modulation
spectroscopy and is used in the stabilization of optical sources. Commercial
lock-in amplifiers that use PSD are often expensive and host a bewildering
array of controls that may intimidate a novice user. Low-cost microcontrollers
such as the Arduino family of devices seem like a good match for learning about
PSD; however, making a self-contained device (reference signal, voltage input,
mixing, filtering, and display) is difficult, but in the end the project
teaches students "tricks" to turn the Arduino into a true scientific
instrument.
