Abstract: An improved switching device for indicating the beginning and ending of the breath of a test subject. A flow through housing is provided with an inlet and first and second outlets. A flexible diaphragm is connected in series with the first outlet to serve as a check valve which opens during the high flow rate portion of a breath. A sensitive flow responsive member connected in series with the second outlet opens in response to the presence of gas flow during the low flow rate portion of the breath. A detecting circuit responsive to the position of the flow responsive member accurately signals the beginning and ending of a breath without interference by the check valve.