Humidity sensor problem

[darvaish]

I am going to use Humidity sensor HS1101 from Humeril.
I studied the dtasheet but its very confusing and i am unable to understand it properly

Have any body ever used this sensor?
I want to know how to use it mean i want the circuit diagram.

A circuit is given in datasheet which converts the capacitance change due to humidity in to frequency.Now by measuring this frequency through a mcu i can find out the relative humidity.but the formulae given in datasheet are very confusing to me
Somebody plz explain them?

Datasheet also talks about the STRAY capacitance.what is it?
also it is said to pay attention to soldering what soldering considerations should be there

 

[joeatxdobs]

I tested the HS1101 in my article http://correctenergysolutions.com/electronics/cap-humidity-sensor-circuit/ I found such a wide variability from sensor to sensor when simply measuring in a capacitance meter than I decided the factory charts are mostly worthless. In my article I describe how to read the sensor with two digital IO lines, 1 resistor and 1 diodes. I also describe how to calibrate the sensor so that it will yield consistent results regardless of how far out of factory spec it is.

On your stray capacitance question, any two parallel sheets of conductive plate such as metal or Mylar can make a capacitor if they are sufficiently close together. We make high voltage capacitors simply by gluing Mylar sheets on both sides of a 1/16" sheet of plastic insulator. They start sparking through the plastic at about 9KV but they hold a charge for minutes and can deliver a large shock to kill bugs. In my humidity circuit logic I count how many cycles in a tight loop it takes to charge the capacitor circuit through a resistor. The count varies based with capacitance. The basic sensor the readings ranged from 1000 to 1350 depending on humidity. When I soldered the sensor onto 3 foot of speaker wire the count went up by about 50 which represents approximately a 5% increase in capacitance. If the sensor was at 240pf then a 5% increase represents a 12pf increase in capacitance in the wire. The capacitance in the wire also changed as a result of the human body (grounding plane?) effect as I moved into and out of range of the wire.

The capacitance effect in the wires is worse than resistance voltage drop from the vout style sensors but with either sensor you want the wire to be short as possible ideally building the micro controller right into the sensor and then feeding the readings back using a digital network such as RS422. The SHT11 and SHT75 use a digital I2C type network but I prefer a network rated for 50 foot wires which I can do easily with RS232 and RS422.

 

[Vizier87]

The capacitance changes with changes of humidity-it's as simple as that. a 555 timer will aid in the usage of that sensor for detection with a microcontroller. The frequency of an astable timer is proportional to it's capacitance, thus it is proportional to the humidity.

See the datasheet, it has a circuit of a 555 timer to manipulate the frequency.
http://www.humirel.com/product/fichier/HS1101-HS1100.pdf

 

[smanches]

All these humidity sensor threads are years old. Some new person just went back through and replied to them all. I would suggest letting them die and starting new ones if you're interested.

 

[Boncuk]

Capacitive humidity sensors require quite some linearization circuitry and calibration.

The SHT11 (Sensirion) is a 14bit combined temperature and humidity sensor precalibrated during manufacturing with a serial output.

Connect to MCU - and done.

Here's the datasheet.

 

[joeatxdobs]

Yes SHT11 is nice but it starts at $25 versus $3.40 for the cap sensor

Capacitive humidity sensors require quite some linearization circuitry and calibration.

The SHT11 (Sensirion) is a 14bit combined temperature and humidity sensor precalibrated during manufacturing with a serial output.

Connect to MCU - and done.

Here's the datasheet.

Yes I have used both the SHT11 and the SHT75 they are nice but they also start at $25.19 each on Newark. I paid $3.43 for my cap sensors so the SHT11 is 730% the cost of the basic sensor. I prefer the more precise SHT75 which is $35. I have found the SHT sensors still require recalibration in less than a year so you must have much of the same logic in the micro controller to store the new offsets and ramps.

I have found most of the CAP sensors provide good linearity but their base offsets are different and change over time. I have received fairly good results with only 2 or 3 calibration points and only needed the 1 diode, 1 resistor, the sensor and the micro-controller. Once you have the software to calculate the calibration and ramp points anyway then the extra cost doesn't seem to buy much. Interestingly my I2C interface and calibration application software for the SHT75 was larger than the software I used to read the cap sensors directly. See: http://correctenergysolutions.com/electronics/cap-humidity-sensor-circuit/ After a very primitive calibration run I normally see readings that after averaging stay within 2% of my nist certified sensor and they agree within 2% of the expected readings for the intermediate salt solutions. I am not sure how much better linearity I need.

 

[mneary]

All these humidity sensor threads are years old. Some new person just went back through and replied to them all. I would suggest letting them die and starting new ones if you're interested.

And every one has a url leading to his design services business.

 

[Boncuk]

Yes I have used both the SHT11 and the SHT75 they are nice but they also start at $25.19 each on Newark. I paid $3.43 for my cap sensors so the SHT11 is 730% the cost of the basic sensor. I prefer the more precise SHT75 which is $35. I have found the SHT sensors still require recalibration in less than a year so you must have much of the same logic in the micro controller to store the new offsets and ramps.

It depends on how much work and material you put in for a circuit to interface with an MCU. (normally limited to 10bit resolution)

I don't have much experience with the SHT11 or SHT75 yet, but I do have experience with differential pressure sensors. They drift along an imaginary zero-line, sometimes to the positive and sometimes to the negative side. The pressure sensors I normally used were made by FURNESS CONTROLS, UK. I just left them uncalibrated for more than five years with no significant change in behaviour of the air handling unit.

I guess having a sensor with 14bit resolution is worth the price saving on cost for the circuit you'd have to design (and apply) for a basic sensor.

Take a look at the Philips humidity sensor. Its capacitance ranges from 114 to 142pf (0 - 100% RH) non-linear.

Boncuk