Abstract: Disclosed herein is a circuit for automatically selecting between an ECG heartbeat signal source and a pressure/pulse signal source depending on which of the two sources is the most periodic between three successive heart beats. A pair of microprocessor internal timers commence a count-up cycle in response to a first detected heart beat from each of the two respective sources, after which count-down cycles are started in response to the next successive detected heart beats. The counting of the timers is then stopped in response to the third successive detected heart beats from the two sources and a comparison is provided to determine which of the two sources is most periodic, and should be used to trigger a heart rate monitor.