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 VCC 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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