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HPP804B100

TEMPERATURE AND RELATIVE HUMIDITY SENSOR

Humirel 

3 TECHNICAL DATA

SUGGESTED EVALUATION CIRCUIT FOR HUMIDITY FREQUENCY

AND TEMPERATURE ANALOG OUTPUTS

Relative humidity (Frequency output)

This circuit is a typical Astable design for 555. The HTS2010 is used as a variable capacitor and provides an output fre-

quency modulation with relative humidity ranging from 0% to 100%.

In this frequency output circuit configuration (for capacitance measurement), R1 unbalances the internal temperature com-

pensation scheme of the 555. It shifts the negative initial temperature coefficient of the 555 to a positive one that matches

the humidity sensing chip’s temperature coefficient of - 1.4Hz/°C. In all cases, R1 should be a 1 % resistor with a maximum

of 100ppm temperature coefficient like all other R-C timer resistors.

Resistors values

Since 555 internal temperature compensation changes from manufacturer to manufacturer, R1 and R2 values should be

adapted to device to be used.

555

R1

TLC555 (Texas)

TS555 (STM)

7555 (Harris)

LMC555 (National)

909kΩ

100nF capacitor

1732kΩ

1238kΩ

For a frequency of 6660Hz at 55%RH

R2

576kΩ

523kΩ

549kΩ

562k

Temperature (voltage output)

The temperature measurement is done by a voltage divider, RV2 must be adjusted to get 2.5 V at 25°C as temperature

measurement.

Polarization

In order to get better reproducibility during measurement, always connect pin 4 (see the package outline on page 4) to the

ground of the circuit.

Soldering instructions

See application note HPC 034 VO

Humirel Sensor Device Data / HPC008 Rev E September 2002

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