We perform phase-sensitive detection with an inexpensive microcontroller, the Teensy 3.5. The programming and pricing of this microcontroller is similar to that of commonly used Arduino microcontrollers, but the Teensy 3.5 offers superior hardware performance. Our Teensy-based phase-sensitive detector can operate either with an external reference signal or by internally generating a reference signal. Additionally, we have developed an open-source graphical user interface for controlling the instrument. We demonstrate that our phase-sensitive detector exhibits good linearity in amplitude and phase, even with 25 mV signals dominated by larger-amplitude noise. We also use our phase-sensitive detector in a simple laboratory measurement: determining the distance dependence of the intensity of a light-emitting diode (LED) with the room lights on. Our instrument is a useful tool for teaching students about phase-sensitive detection and can be a viable low-cost alternative to commercial lock-in amplifiers.