Bidirectional subminiature thermistor sensor for analog control by breath flow

A subminiature airflow sensor is described, which can discriminate between exhalation and inhalation, has good linearity and stability, and is small enough to fit inside the stoma button of a laryngectomee. The system employs two fast-response self-heated thermistors placed on opposing sides of an aerodynamic obstacle inside a tube; flow polarity discrimination is based on differences between the heat exchange patterns under laminar vs. turbulent flow. An output proportional to airflow is achieved by the connection of a circuit with a piecewise-linear transfer characteristic in cascade with the thermistors. Developed in-house, this circuit may be adjusted to create an arbitrary range of input-output characteristics. Applications are suggested for an input device for various handicapped aids, for medical diagnostics and in industrial control.