NTC temperature inspection instrument.Pyroelectric infrared sensor module

　　Temperature monitoring is mainly achieved using temperature sensors. The temperature sensor in this design adopts NTC thermistor, that is, thermistor with Negative temperature coefficient, whose resistance value (RT) decreases rapidly with the increase of temperature (T). The expression for the resistance temperature relationship is:

　　LTC1799 is a resistance programmable oscillator that can generate square waves with a duty cycle of 50% and has characteristics of temperature stability and power supply voltage stability. It is a low-power device that only requires one peripheral component, namely a resistor and bypass capacitor. The LTC1799 standard circuit is shown in Figure 1, with 0.1 in the figure μ The capacitor of F is connected between the power pin and ground, which can minimize power noise. A resistor is connected between pins 1 and 3 to control the output frequency. In this design, a thermistor is used instead of setting the resistor. The fourth pin is a three state division pin that determines whether the master clock is divided by 1, 10, or 100 before output. In the design, this pin is grounded, which means the output division coefficient is 1. The 5th pin is the output pin, and the relationship between the output frequency and the set resistance is:

　　As the resistance value of the thermistor changes with temperature, the relationship between temperature and frequency can be established through LTC1799 to achieve temperature measurement.

　　According to equations (1) and (2), there are two types of nonlinear relationships in circuit design: one is the nonlinear relationship between the resistance value of the thermistor and temperature, and the other is the nonlinearity during the frequency conversion of the resistance value. For nonlinear problems, mathematical methods can be used to handle them, but the algorithm is relatively cumbersome and requires a large amount of hardware resources. Therefore, another method was adopted in the design for processing, which is to use ROM to pre store frequency temperature data and use table lookup method for temperature mapping. This not only avoids the impact of nonlinear problems but also saves hardware resources.

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