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.