Remote humidity sensor needs no battery

Using ac-line power sources and batteries for remote humidity sensors is undesirable because these sources can be troublesome if you mount them in inaccessible points, such as smokestacks, cold-storage chambers, or darkrooms, where maintenance is difficult and inconvenient. Figure 1 shows a simple way to remove the power source from the humidity-sensor circuit. The circuit uses a 160- to 200-pF, capacitor-type HS1101 humidity sensor from Humirel for the circuit. IC₁ forms a classic oscillator of time constant R₁C₁. The center frequency is 5 to 10 kHz, depending on the value of R₁. The circuit charges the 100-µF, low-leakage capacitor, C₂, through the diode, D₁, directly from the output line of the sensor oscillator, IC₁. C₂ becomes the power source for IC₁.

When you make measurements, you stop charging C₂ for a short time by removing the Control Input signal (0V for charging and V_CC or floating for measurement). You then measure the frequency of the signal at the output of transistor Q₁. Transistor Q₂ supplies enough current to charge C₂ through R₄. The HS1101 sensor measures approximately 160 pF at 0% relative humidity and 200 pF at 100% relative humidity. Therefore, frequency decreases with increasing relative humidity. Although the sensor has a linear response within ±5% of full-scale range over 1 to 25°C, you should calibrate the circuit at several humidity points. You can find more details about the HS1101 sensor at www.humirel.com or www.sensorstechnology.tripod.com/humidity.html.

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