Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

humidity sensor

Status
Not open for further replies.

mckooiker

New Member
I would like to create a circuit (low voltage; dc) to keep a small chamber free of humidity, using a pc-cooling-fan for that and two humidity sensors: one in the chamber and one outside the room. The idea is to turn the fan on, only if the humidity inside the chamber is higher than outside.

I thought I'd figured something simple out with two resistive humidity sensors and a wheatstone bridge, but then I read that you'd polarise the sensor if you use DC instead of AC voltage, reducing the life of the sensors..... I wouldn't know where to start with a capcitive sensor.

Though I suspect it should be relatively simple to build the circuit: unfortunately I'm a noob, therefore I as your help!
Thanks a lot in advance,
Maarten
 
Last edited:
Hi Maarten,
First of all Welcome to the forum :)
Before the days of electronics the way to measure humidity was a temp gauge dry and a temp gauge wet, the difference was worked out to be the humidity. Do some research on that concept and a project will soon bear out for your application.

Regards Bryan
 
Last edited:
Thanks Ron,
Actually when I first read this post I went and asked my missus how they used to measure humidity before the day of transistors. That link will be great for Maarten and also myself as I did wonder how they worked......

Thanks

Regards Bryan
 
i use the honeywell 5031 sensors, there very easy to use and are analogue

you can either use ADC (if you can do programming)

or make some kind of comparator.
 
You could feed the outputs from two of the 5031 sensors into a comparator, one to the (+) input and one to the (-) input. You probably want to put an adjustment pot on the output of the one inside the chamber so you can adjust the comparator's off/on point to be slightly below the chamber humidity, otherwise the fan may never turn off. Also you want to add some adjustable hysteresis feedback to control the difference between the on and off points.
 
Hi Maarten,
First of all Welcome to the forum :)
Before the days of electronics the way to measure humidity was a temp gauge dry and a temp gauge wet, the difference was worked out to be the humidity.

Objections your honor! :)

That method is still used at meteorological stations to determine the dew point.

Regards Bryan[/QUOTE]
 
If you have to employ an MCU why not use a 16-bit serial sensor like the SENSIRION SHT11?

Alternatively you might use an SHT75, which is manufactured with a 4-pin connector, costing 10 EURO more than the SHT11.

The SHT75 is €36.77, the SHT11 is €26.44.

A cheaper sensor (requiring extra circuitry for temperature compensation and a 0 to 10V output) is the EFS-10 for €7.42 (data sheets for all three sensors are available at Conrad electronics. The EFS-10 data sheet contains a full circuit for above mentioned output criteria.

You might source all of them at Conrad - Online Shop für Technik, Elektronik und innovative Ideen

It combines temperature and relative humidity measurement. The sensors are calibrated during manufacturing - so no calibration as necessary with an analog sensor.

I use the SHT11 sensor with an incubator circuit and it works as accurate as a high class (and expensive) hygrometer.

Boncuk
 
Last edited:
thanx

again everyone....
Unfortunately the room is too small for the first solution, and I would like to keep humidity as much out of it as possible.
The idea of crutschow summarises more or less what I had in mind and I think I will have a go on that one...
I'll keep you guys updated on the proceedings!
Cheers,
Maarten
 
Just another idea... if you did want to go with the resistive sensors (maybe cheaper?), you can just add an AC differential drive to your bridge, connect the comparator (or opamp) to the sense points on the bridge as usual, and feed the comparator output and one of the drive lines into an XOR gate to give you your desired output. See attached cct. (note the transistor is being used as the XOR gate) & sorry for the bad quality - hopefully you can read it.
 

Attachments

  • ac humidity.jpg
    ac humidity.jpg
    50.8 KB · Views: 178
thanks dougy

If you would have read my initial post: I'm a noob, so I'm affraid I need some clarification (the word cheaper cfully caught my atention ;) ).

I get the general idea of the circuit, but I'm a bit puzzled on how the circuit is going to switch the direction of current (change to AC from DC).

I guess the trick is in the triangular elements (which I dont know what they are; you got 3 of them in your circuit, two of them on the left and one on the very right).

Cheers,
Maarten
 
I haven't verified its operation but the circuit apparently is a phase sensitive (synchronous) detector. As the bridge goes unbalanced from one direction to the other, the AC bridge output signal changes phase by 180 degrees. An Exclusive-OR circuit can act as a synchronous detector if one input is the AC source and the other is the AC bridge signal. The Exclusive-OR output will be zero (or negative) if the signals are in-phase and high if the signals are out-of-phase.

The triangular elements are digital Schmitt-trigger logic inverter gates.
 
(the word cheaper cfully caught my atention ;) ).
Yeah, it does that. I don't even know if it's the case - I'm just assuming the resistive sensors would be cheaper than the 5031s.

I get the general idea of the circuit, but I'm a bit puzzled on how the circuit is going to switch the direction of current (change to AC from DC).
The triangle (schmitt trigger inverter - thanks crutschow) on the left oscillates at a freq determined by the R & C connected to it. The second inverter inverts that voltage. So you get AC across the bridge. The output of the opamp will be changing state at the same rate as the oscillator/AC voltage - but depending on the values of the resistors in the output will be either in-phase or out-of-phase with the oscillator. crutschow explained the XOR phase detector better than I can. The output of the XOR has short glitches due to the slowish rise/fall times of the opamp and these are removed by the 100k res/1u cap low pass filter before being fed into a final schmitt trigger to clean up the signal.

As for verification of the xor part - the attached simulation says it's ok. I think the rest should work.

PS the diode shown on the original cct probably isn't required and should be removed if the supply voltage is low e.g. <= 5V.
 

Attachments

  • xor.gif
    xor.gif
    15.5 KB · Views: 153
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top