The heartbeat monitor has a housing containing a supply battery, reception and amplification circuits and a counter circuit coupled to a read-out display (22). The housing has a removable back plate and the front surface has a recessed zone (16) incorporating a viewing window (20) for the fluidic crystal read-out display (22). The opposite side of the front surface has a depression (24) extending inwards from one side edge, the same width as a finger, the base of which has a transparent plate covering a lamp (28) and a plethysmographic pick-up (26). The finger depression (24) has a rounded end and is of sufficient depth for the lamps and the pick-ups to be completely covered when the finger is placed in the depression, when the ingress of external light is blocked.
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