Abstract: The present invention discloses a pneumatic, mechanical, pulmonary ventilator apparatus. The apparatus operates to dilute and limit the pressure of various gases delivered to a patient. The function of the apparatus may be described as an astatic multi-vibrator, which is activated by pressurized gas (e.g., such as oxygen or medical compressed air). The gas output of the apparatus may be described as a square wave, wherein the high wave phase corresponds to the inspiratory phase (the duration is the inspiratory time) and the low wave phase corresponds to the expiratory phase (the duration is the expiratory time). The sum of the two phases corresponds to the respiratory cycle. The device includes a rubber diaphragm having a ring shape to cyclically block a gas source aperture inlet. An adjustable needle valve, which controls the gas pressure in a small chamber on the opposite side of the diaphragm, together with a helical spring provide for tuning the two cycle phases.