This Letter presents a novel, wrist-worn heart rate sensor, which is comprised of only a passive resonator and a voltage controlled oscillator (VCO). It is clearly demonstrated that valid heart rate data can be extracted off the DC bias circuit of the VCO, making this sensor one of the simplest of its kind ever reported. Experimental results are shown for successful measurements of the user's heart rate when the VCO was tuned to 2.42 GHz, with a DC bias voltage fluctuation of up to 10 mV representing the heart rate. These results are in agreement with those taken using a commercial optical sensor based heart monitor.