Datasheet HS1101LF (TE Connectivity) - 4

HS1101LF Relative Humidity Sensor This circuit is the typical astable design for 555. The HS1101LF, used as variable capacitor, is connected to the TRIG and THRES pin. Pin 7 is used as a short circuit pin for resistor R4. The HS1101LF equivalent capacitor is charged through R22 and R4 to the threshold voltage (approximately 0.67Vcc) and discharged through R22 only to the trigger level (approximately 0.33Vcc) since R4 is shorten to ground by pin 7. Since the charge and discharge of the sensor run through different resistors, R22 and R4, the duty cycle is determined by: 
thigh = C@%RH*(R22+R4)*ln2

tlow= C@%RH*R22*ln2
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F = 1/(thigh+tlow) = 1/(C@%RH*(R4+2*R22)*ln2)

Output duty cycle = thigh*F = R22/(R4+2*R22)
To provide an output duty cycle close to 50%, R4 should be very low compared to R22 but never under a minimum value. Resistor R3 is a short circuit protection. 555 must be a CMOS version
TYPICAL RESPONSE LOOK-UP TABLE RH (%) 0 5 10 15 20 25 30 35 40 45 50 Fout (Hz)
- - 7155 7080 7010 6945 6880 6820 6760 6705 6650
RH (%) 55 60 65 70 75 80 85 90 95 100 Fout (Hz)
6600 6550 6500 6450 6400 6355 6305 6260 6210 - MEASUREMENT ERROR VS STRAY CAPACITANCE Special attention is required in order to minimize stray capacitance in the layout. The added capacitance will act as a parallel capacitance with the sensor and create a measurement error. A careful coating of PCB and components must be implemented to prevent unexpected deviations of Fout in high humidity conditions. SENSOR SOLUTIONS /// HS1101LF HPC052_J 9/2015 Page 4 Document Outline HS1101LF Relative Humidity Sensor