Humidity sensor operation

Thread Starter

skywest

Joined Jul 16, 2014
4
Hi,
I am trying to design a simple system that measure the humidity in my car and switches on a small fan when it reaches a threshold, powered by say a 9v battery, not by the car battery.
So I am trying to research humidity sensors and really can't find out too much about the implementation and operation.
One such is http://www.spiratronics.com/data/8315.pdf but the data sheet doesn't give any info other than electrical characteristics.

Can anyone point me in the right direction please?
 

ericgibbs

Joined Jan 29, 2010
12,970
hi sky,
That sensor needs extra components in order to convert the RH% resistance at a particular temperature to a readable RH value for the user.

Do you have any electronics equipment and knowledge.?
E
 

LesJones

Joined Jan 8, 2017
3,195
I think the simplest method is to use a DHT22 with a small microcontroller. I assume when you say a 9 volt battery you mean a PP3 type. Based on that assumption you would need to only power the electronic for a short time say every minute to conserve the battery. (As humidity will not chance rapidly.) The microcontroller could be programed to be in sleep mode for most of the time.
An alternative sensor is the BME280 which is more accurate but requires more complex software in the microcontroller.

Les.
 

AlbertHall

Joined Jun 4, 2014
11,317
Using a microcontroller is the simplaest if you have the knowlwedge and, possibly, hardware to program one.
If you have never done this before it is far from trivial.
 

wayneh

Joined Sep 9, 2010
17,152
Hi,
I am trying to design a simple system that measure the humidity in my car and switches on a small fan when it reaches a threshold, powered by say a 9v battery, not by the car battery.
So I am trying to research humidity sensors and really can't find out too much about the implementation and operation.
One such is http://www.spiratronics.com/data/8315.pdf but the data sheet doesn't give any info other than electrical characteristics.

Can anyone point me in the right direction please?
If I were trying to do this, I'd pull the sensor and controller from a dead home dehumidifier. These things fail after one season and there are hundreds out there attainable for free. Every part of them works fine but the freon is gone. Get one of the newer ones with the digital readout and digital controls. The old kind use an old-fashioned humidistat sensor. Although now that I think about it, maybe that's exactly what you want. It can accomplish the switching with no electronics or power supply.
 

Thread Starter

skywest

Joined Jul 16, 2014
4
If I were trying to do this, I'd pull the sensor and controller from a dead home dehumidifier. These things fail after one season and there are hundreds out there attainable for free. Every part of them works fine but the freon is gone. Get one of the newer ones with the digital readout and digital controls. The old kind use an old-fashioned humidistat sensor. Although now that I think about it, maybe that's exactly what you want. It can accomplish the switching with no electronics or power supply.
Thanks, good idea, I'll try that.
 

Reloadron

Joined Jan 15, 2015
5,935
For a clean dry environment I would go with Les Jones suggestion using a DHT 22 sensor and as can be seen they are very common and inexpensive. I would use it together with a uC (Micro-Controller) allowing me to easily create a set point. I would pair it with an inexpensive Arduino Uno Clone from Amazon like this one. I would just power the uC board off the automotive 12 volt system and the DHT 22 off the Arduino 5 volts out. If anyone wanted to get fancy just add a LCD.

DHT 22 1.png

Everything modular making for easy interconnect. Including plenty of sample code online.

Just a matter of If RH is greater than .... Do something or whatever one wants to happen.

Ron
 

bobcroft

Joined Aug 22, 2011
11
The DHT22 may be cheap but with that comes unreliability, it will do for a basic humidity switch though. The BME280, although more expensive, is both more accurate and reliable and will give you temperature and barometric pressure as a bonus!. In terms of using them with, say, an Arduino microcontroller there is no real difference, since both use freely available libraries. Have a look at the www.randomnerdtutorials.com website, it is an excellent learning source for beginners and there will be suitable tutorials on both the DHT22 and the BME280. As someone else said you can power the Arduino from the car nominal 12 v supply but you must power the sensor from the Arduino's regulated 5 or 3,3 volt supply. Make sure that the Arduino is in a well ventilated to keep it cool.
 

Phil-S

Joined Dec 4, 2015
205
The Honeywell HIH-4000 RH sensors are as simple as they come with just power (5-volts) and an analogue output.
Honeywell has a good reputation. The sensor does not require complicated calibration.
Whatever solution you go for, be it seaweed to specialized sensors, you are going to have to do some interfacing with possibly electronics like op amps or transistors.
Microcontrollers are the long-term best solution for dealing with inputs and outputs, but there is a huge amount of learning for maybe just one application. On the other hand, you might suddenly discover the usefulness of MCUs.
Having said that, I control the ventilation in a roofspace by using an Xpelaire humidity fan controller, much the same as the Ranco controller mentioned before.
For monitoring purposes, though, I use the HIH-4000 on a 1-Wire bus.
Curious why humidity is important in a car?
Does it leak a lot or are you trying to grow things?
My first cars didn't need sensors, your wet feet told you all you wanted to know.
 

Reloadron

Joined Jan 15, 2015
5,935
Hopefully during April and to date in May the thread starter has something going by now. Next with any of the devices mentioned and it matters not be it barometric pressure, temperature or relative humidity the end user find a way and means to calibrate the measuring device or UUT (unit under test). You need a standard which has a greater accuracy that your UUT. With humidity a saturated salts slurry is a good start and Google is your friend. This only holding true if you want really accurate measurements. If you can live with a larger margin of error then don't worry about calibration.

Ron
 
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