Unobtrusive vital sign monitoring is essential for most wearable monitoring applications. This paper presents a new method for unobtrusive vital sign monitoring based on the change in capacitance of passive sensors in contact with the user's body. Unlike traditional capacitive coupling that is commonly used for elimination of the DC component of bioamplifiers, we propose monitoring of heart rate using galvanically isolated capacitive sensors. This paper presents the theory of operation and the implementation of a prototype device for heart rate monitoring. A prototype capacitive sensor was implemented using parallel coplanar plates in contact with the tip of the finger. Preliminary testing allowed us to observe changes in capacitance in the range of 0.1-0.3 pF per heartbeat, or 0.3-0.9% of the measured signal. The detected heart rate was validated using a PPG sensor positioned on the same hand. During a single recording session of 772 s, the detected heart rate was within 3 BPM 42% of the time. Proposed method is sensitive to motion artefacts; however, processing of signals collected from multiple contact points may facilitate removal of motion artifacts. Since many microcontrollers directly support high quality monitoring of capacitance on multiple I/O pins, the proposed method provides a promising solution for a low-power, low-cost, multi-contact monitoring.