# Electronic Humidity Sensor

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#### Vizier87

##### Active Member
Hi guys... I've designed a low cost humidity sensor based on previous suggestions using a carbon resistor with its coating removed. Here's the schematic I want to build:

http://www.electro-tech-online.com/attachments/hygrometer-jpg.32267/

Has anybody done this before? Do I need to introduce other stabilizing parts like temperature compensation, etc.? The output it not very linear, so I plan to calibrate it using programming to linearize its output.

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#### DexterLB

##### New Member
Great Idea! Unfortunately I haven't made something like that but I want to.
I prefer AVR instead of PIC, I can use AtMega8 which has a pretty good ADC.

But how to do the calibration? You'll need a working very accurate humidity meter and controlled environment without air currents and a way to gradually change the humidity... Now that'll be difficult. You (and I) will also need to use accurate resistors, say, like, 0.5, max 1%

#### Boncuk

##### New Member
Carbon film resistors change resistance considerably with temperature changes.

There were experiments done using 0.1% metal film resistors, but after some time the measurement becomes unreliable because of oxydation of the metal film.

Philips (NXP) offers a reasonably priced (€10) humidity sensor using a gold cap of 122pF initially changing capacitance 0.4pF/%RH. This also non linear.

Boncuk

#### Externet

A defunct VCR can donate a humidity sensor for $0. It is located near the tape path, it is about 1cm x 1cm x 1mm, has two wires. #### Vizier87 ##### Active Member Carbon film resistors change resistance considerably with temperature changes. There were experiments done using 0.1% metal film resistors, but after some time the measurement becomes unreliable because of oxydation of the metal film. Philips (NXP) offers a reasonably priced (€10) humidity sensor using a gold cap of 122pF initially changing capacitance 0.4pF/%RH. This also non linear. Boncuk Thanks for the info. Are there links to the 'dissertation'? How about if the filming of the resistors use more oxide-resistant materials? A defunct VCR can donate a humidity sensor for$0. It is located near the tape path, it is about 1cm x 1cm x 1mm, has two wires.
is the sensor capacitive or resistive? Also, do they have it in radio cassette players? Coz that- I have a lot of broken ones back here in M'sia.

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#### DexterLB

##### Banned
I'm gonna guess capacitive. They simply much more common.

#### Boncuk

##### New Member
Here is a real good humidity sensor.

It's combined temperature/humidity, calibrated during manufacturing and has a serial 16bit output.

I will use it in an incubator circuit. The PCBs are already ordered and the prototype should be finished and tested by the end of next month.

The sensor is a "Sensirion" SHT11, measuring 5.08X7.62mm

Here is the package design for Eagle.

Boncuk

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