A vein locating device for locating a vein on a patient has a housing and an objective lens. A condenser lens is disposed within the housing and is aligned with the objective lens along a common axis. There is an infrared light source for emitting infrared light from the vein locating device onto an area of skin of the patient. There is a sensor circuit which includes a first photosensor, a second photosensor, a comparator, and a marking light.
