air flow sensor circuit

Posted on Feb 6, 2014

This circuit is describes about sensing air flow using microcontroller PIC16C781. In this circuit is using Programmable Switch Mode Controllers (PSMC) that combination between the Integrated Operational Amplifier, Digital-to-Analog Converter (DAC), and gated timer to construct a thermally operated air flow sensor with minimum external components.

air flow sensor circuit
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This is the figure of the circuit. Air flow is detected by the cooling effect of air movement across a heated resistor. R5 and R7 are thin film platinum Resistance Temperature Detectors (RTD). These are essentially thermistor with a very linear temperature response. The flow sensor is comprised of R6 and R7. Changes in ambient temperature conditions are compensated by two voltage dividers, R2-R5 and R1-R7. R2 and R5 form a voltage divider between the Op Amp output and the Op Amp inverting input. Similarly, R1 and R7 form a voltage divider between the variable DAC reference and the non-inverting Op Amp input. Since R5 and R7 are identical RTD`s, resistance variations due to self heating, as well as changes in the ambient conditions, cancel out at the Op Amp inputs. [Schematic source: Microchip Technology Inc].

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