NTC thermistor linear control circuit for fan speed based on room temperature.Pyroelectric infrared sensor module

　　Operational amplifier A1 is essentially used as an I to V (current to voltage) converter and converts temperature changes into voltage changes. In order to amplify the voltage changes caused by temperature changes, an instrument amplifier composed of operational amplifiers A2, A3, and A4 was used. The combination of resistor R2 and Zener diode D1 is used to generate a reference voltage because we only want to amplify the voltage changes caused by temperature changes.

　　operational amplifier μ A741 (IC2) is used as a comparator. One input of the comparator is the output of the instrument amplifier, while the other input is AC voltage sampling that has undergone voltage reduction, rectification, and appropriate attenuation. This is a negative pulsating DC voltage. It can be seen that as the temperature increases, pin 2 of IC2 becomes increasingly negative, so the width of the forward output pulse (pin 6) increases linearly with temperature. Therefore, IC2 is used as a pulse width modulator in this circuit. The output of the comparator is coupled to an optocoupler, which in turn controls the AC power supply supplied to the fan (load).

　　This circuit has high sensitivity and the output RMS voltage (across loads) can vary between 120V and 230V (for temperature ranges of 22 ° C to 36 ° C), resulting in significant speed variations. Also note that the speed changes linearly, not gradually. In addition, due to the use of optocouplers, the control circuit is completely isolated from the power circuit, thereby increasing safety. Note that for any given temperature, the fan speed (i.e. voltage on load) can be adjusted to the desired value by adjusting the potentiometers VR1 and VR2 appropriately.

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