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.
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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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