The Type NH-02 humidity sensor from Figaro consists of a capacitive humidity sensor, Zs. in series with a thermistor.Z, on an aluminum substrate. The parameters of the thermistor ensure that it compensates the temperature-dependence of the dielectric of the sensor almost completely. Over the temperature range 15-35 °C. the temperature dependence of the NH-02 is about 0.3% K-1.
Before the sensor can operate. it needs to be energized by an alternating voltage with a frequency of 50-1000 Hz. In the present circuit, that signal is generated by a Wien oscillator based on IC1a, whose frequency is set to about 1 kHz by bridge R3-R4-C3-C4. The two diodes in the feedback loop ensure amplitude stability. The level of the alternating signal, and thus the maximum Output voltage, can be set over a wide range with p2. This preset is coupled to the sensor by buffer IC1b, which prevents the impedance of the sensor influencing the set voltage. The output voltage of the sensor is buffered by IC1 and then applied to peak rectifier ICid-D3-D4. The output voltage of the rectifier is smoothed by R10-C6. after which the direct voltage is amplified x5 by IC2b (a = 1+Ri2/R11).
The characteristic of the relative humidity vs the output voltage is not linear as shown in the graph. For simple on-off switching applications that do not matter, but for others, it may be necessary to make the voltage linear. This is best done with a computer and suitable software.
The power supply may be an asymmetric stabilized 8-15 V type. Opamp produces an artificial earth at half the supply voltage so that the circuit is powered by a symmetric 4-7.5 V supply. Note that the supply voltage must be larger than the wanted output voltage.
The circuit is best aligned with the aid of an oscilloscope. With P1, set the amplitude of the oscillator to a peak value of 2 V (measured between pin 1 of IC1 and earth). Then with P2, set the wanted peak value of the supply to the sensor. In many cases, a peak value of 1 V will be found suitable: the output voltage then varies between 0 V and 5 V.