HPP801A031 Ver la hoja de datos (PDF) - Humirel

Número de pieza

componentes Descripción

Fabricante

HPP801A031

RELATIVE HUMIDITY SENSOR

Humirel 

FREQUENCY OUTPUT CIRCUITS

Note: R22=499kΩ / R4=49.9kΩ / R1=1 kΩ / RV1=50 kΩ potentiometer / C1=10nF / C2=2.2nF / C3=100nF

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:

¾ thigh = C@%RH*(R22+R4)*ln2

¾ tlow= C@%RH*R22*ln2

¾ F = 1/(thigh+tlow) = 1/(C@%RH*(R4+2*R22)*ln2)

¾ 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

Fout

(Hz)

7155 7080

RH

(%)

55

60

65

70

Fout

(Hz)

6600

6550

6500

6450

20

7010

75

6400

25

6945

80

6355

30

6880

85

6305

35

6820

90

6260

40

6760

95

6210

45

6705

100

50

6650

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.

HUMIREL HPC052 Rev.E April 2007

TECHNICAL 3 DATA

HS1101LF

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