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2014-11-14 12:52  
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The circuit is the typical astable design for 555. The HS1100/HS1101, 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 HS1100/HS1101 equivalent capacitor is charged through R2 and R4 to the threshold voltage (approximately 0.67Vcc) and discharged through R2 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, R2 and R4, the duty cycle is determined by :thigh = C@%RH*(R2+R4)*ln2tlow = C@%RH*R2*ln2F = 1/(thigh+tlow) = 1/(C@%RH*(R4+2*R2)*ln2)Output duty cycle = thigh*F = R2/(R4+2*R2)To provide an output duty cycle close to 50%, R4 should be very low compared to R2 but never under a minimum value. Resistor R3 is a short circuit protection. 555 must be a CMOS version. REMARK R1 unbalances the internal temperature compensation scheme of the 555 in order to introduce a temperature coefficient that matches the HS1100/HS1101 temperature coefficient. In all cases, R1 should be a 1% resistor with a maximum of 100ppm coefficient temperature like all other R-C timer resistors. Since 555 internal temperature compensation changes from one trademark to one other, R1 value should be adapted to the specific chip. To keep the nominal frequency of 6660Hz at 55%RH, R2 also needs slight adjustment as shown in the table. HS1101HS1101

                                       For a frequency of 6660Hz at 55%RH