  The CosmicWatch Desktop Muon Detector is a self-contained, hand-held cosmic
ray muon detector that is valuable for astro/particle physics research
applications and outreach. The material cost of each detector is under $100 and
it takes a novice student approximately four hours to build their first
detector. The detectors are powered via a USB connection and the data can
either be recorded directly to a computer or to a microSD card. Arduino- and
Python-based software is provided to operate the detector and an online
application to plot the data in real-time. In this paper, we describe the
various design features, evaluate the performance, and illustrate the detectors
capabilities by providing several example measurements.
