Is there a cheap humidity sensor?

[oPiaNts]

I will be working on a project wherein the amount of moisture in the air will be a factor. I have been looking for various humidity sensors but they are too expensive.

Anyone know of a cheap humidiy sensor like and LM35 or thermistor for a temp sensor?

Thanks in advance

 

[Paul Obrien]

you can make your own, get two matched(as close as possible, get a container with two equal sized chambers (make from pc board) have the two resistors mounted equidistant from the walls. Drill a small hole into ONE of the chambers. Apply a constant current to both resistors(in a bridge config) and the difference in voltage across the two resistors will be due to humidity.

 

[oPiaNts]

you can make your own, get two matched(as close as possible, get a container with two equal sized chambers (make from pc board) have the two resistors mounted equidistant from the walls. Drill a small hole into ONE of the chambers. Apply a constant current to both resistors(in a bridge config) and the difference in voltage across the two resistors will be due to humidity.

sorry but huh?

 

[giaosucan]

make humidity sensor by yourself???Are you sure the quality of it???

 

[mramos1]

Sorry Paul, I think we need pictures on this one. I am lost at matched? Matched resistors?

One R sealed air tight and one R sealed, but with a hole in housing? In parallel right? Meter on the one with the hole, breath on it and see the voltage a change? I will try it, calibrating it might be fun..

 

[fingers]

Not sure whether you were looking for a component to add to your circuit, or if you juse wanted to check the humidity whilst doing the experiment. But RS sell cheap Hygrometers www.rswww.com RS# 408-6109 £16.90 each. I've installed these ones at work to monitor RH.

 

[Oznog]

**broken link removed**

The sensor itself has a variable capacitance. They have a spec sheet with a 555 circuit to convert that to freq for a uC. If you want a voltage output you need one of the modules.

 

[mramos1]

Looks like they are expensive, and to save money you pay in time and parts..

http://www.discovercircuits.com/S/s-humidity.htm

interesting **broken link removed**

Also, I saw no cost listed.. **broken link removed**

 

[Nigel Goodwin]

Also, I saw no cost listed.. **broken link removed**

I've got one of those somewhere?, I got it as a free sample - the plan was to use it for another tutorial, but I've not got round to it yet!.

 

[mramos1]

I did not see a price, OK on that page. But they did RH and temp..

Found a site that had the sht10 for $10.50 US, so a little better.

 

[BeeBop]

Yes, I have seen the sht1x on www.TechToys.com.hk for just over $10 US, which seems like a good deal. I have also used the Humirel sensor, but they are about the same amount. Seems like the going rate for an accurate humidity sensor is about ten bux.

I have also heard about using a resistor with the outside layer removed to expose the carbon, to measure, and an op amp to scale, the reading. I haven't had a chance to try it, but it sounds like it would work. (I think Paul's idea is similar.) I have removed the outside coating on a ceramic (1 w?) resistor, and it was easy to do. The problem with trying this method is that you would have to calibrate it yourself. Once done, however, it would indeed be cheap.

 

[mramos1]

Beebop... I like you thought cheap, cool, count me in on a try at it.... I was waiting on Paul to get back and comment.

But my day did not allow doing it.. I would think crack the epoxy off one resistor and then match with another (cracked one in the tube with the air hole). Not sure why the match though.. Maybe to make calibration easier? But something to do.. (As Nigel shakes his head and says "$10 people for the sht10, come on").

Maybe next weekend.

 

[BeeBop]

I think the idea of having two, is so that one can be used as a reference.
However, I was just measuring a resistor with the outer coating removed, and didn't see much change between when I measured it in the room, oh!, just a minute...
I measured 130 ohms before I tried changing humidity, and around 122 ohms in water. (for sure 100%) and then with the resistor sandwiched between a couple of silicon bags (you know, the ones to keep electronics dry,) and now am reading 187 ohms. It is still drying out, now 192 ohms.
My conclusion is that this is do-able. I see my ohm meter is now reading 206 ohms. I pulled the resistor out of the silicon bags, and the ohm meter started to drop immediately, so it responds fairly quickly, but perhaps not as quickly as a thermistor.
I don't think it will be a linear change, but it would be a cheap way, and if you were planning on doing this a number of times, then the work to scale it would be reusable.
Something else: I just applied water to the resistor again, and my reading is 111 ohm, so some error in measurement. Perhaps the battery in my ohm meter is not in good shape, but I think this is a workable solution.

 

[mramos1]

Beebop:

122 in water (first time; 100% for sure, I trust you ). Then 111 (second time; same tank of water I take it?).

OK, tolerance is off a bit. hehe. Maybe bad water the second time (kidding).

But it works for something.

And I have many DVMs due to the fact some read better at some things and batteries get low all the time..

 

[BeeBop]

Yes, this was my cheap, $6.00 job, which works not too badly. Sometimes I wish I had my old analog meter, which seemed to be much more responsive, and perhaps accurate, than the digital ones.
The important thing is that it does work, and is worth pursuing.

 

[mramos1]

BeeBop: I agree.. I was gonna crack on AllVol for pictures of an analog meter in a drawing to someone (to check a POT) yesterday and I said "no, do not do it". I miss mine too.

Weird, the needle tells more than a randon number on a display. I used to have a Simpson 360 or something like that. For leaky parts and current they are the best..

 

[joeatxdobs]

I recently used the HCH-1000-002 which I purchased for $3.43 qty 25 from Mouser. This is a capacitive sensor. I posted an article on how to read these using two digital I/O lines, 1 resistor and 1 diode. No ADC needed. The readings are pretty close in both accuracy and variability to the vout HTM1500 sensors. Calibration is required but is also fairly easy. Additional sensors can be added and only need 1 additional I/O line per sensor. See: http://correctenergysolutions.com/electronics/cap-humidity-sensor-circuit/ for schematic and Bascom AVR source. I tested it with a Mega 16 but it is small enough to install in 4K chips. If re-written in C I think it would fit into a 2K chip.

 

[paulmandel]

Anyone know of a cheap humidiy sensor like and LM35 or thermistor for a temp sensor?

DHT11 One wire solution. About $2.00USD or less Temp in C and Humidity.

Paul

 

[Boncuk]

This is a real cheap one.

Use a horse tail hair and fix it to a board. Connect a spring to the other end to keep it under small tension. Attach some gear to the hair to move an indicator needle. The swing of the needle tells you the RH.

Boncuk

 

[OlPhart]

Howdy, been here, what's your time worth? As versed accuracy? Buy the module if only to have simpler, reproducible results. Simple answer: cheap humidity sensor will have absurd accuracy, commercial units not so bad... G.H. <<<)))