Abstract: A control circuit for a portable radio or like entertainment device which allows it to play only so long as the user's heart rate lies within a range commensurate with safe, effective exercise activity. EKG signals are picked up by suitably placed electrodes, amplified, filtered, shaped and applied in one arrangement to a programmable microprocessor and in another arrangement to a linear circuit phase lock loop and window comparator. In each instance, a determination is made as to whether the user's heart rate is between a lower limit of effective exercise and an upper limit of safe exercise. When the heart rate is outside the limit, the entertainment device is rendered inoperative, thus providing an incentive to tailor the level of exertion so that the heart rate will lie within the desired range.