Fans turn on when Humidity is different between two humidistats

Hey,
First time posting anything. Curious to learn something new.
I was wondering if this was possible. I want to use two humidistats.
If the difference between the two humidistats is greater than 2% then the fans will kick on.
Maybe 2-4 fans.

I have never built anything electronic before. So I'll be honest, I have no clue what I'm doing.
I can sorta understand some of the diagrams I've seen here. But I'm willing to learn.

Any suggestions?
Thanks everyone!!!

The concept sounds possible but there are a lot of things to consider.

You would need humidity sensors, not humidistats, because a humidistat, with an on/off output won't tell you the humidity level.

So you need two humidity sensors, with output levels that can be compared. They need to be accurate enough that they give readings much less than 2% different when the fans have done their job and equalised the humidity.

You needs some way of comparing the outputs of the two sensors, and when the difference is greater than 2%, activating an output.

You need something to make that output turn on the fans that you have. That will probably be a relay.

You need something to stop the fans turning on and off very quickly. The obvious thing to do there is to turn on the fans when the difference is 2% or more, but not turn them off until the difference is less than say 1.5%

There may need to be a test for useful operating range. If the humidity is really high or really low, the sensors may not be accurate, in which case you might want to have a different behaviour if the humidity is very high or very low.

Then you need stuff like power supply, box, displays so you know it's working and so on.

I've not done much with humidity sensors, but 2% seems to be a very small difference, due to the inaccuracies of measuring humidity. I had a quick look at humidity sensors that RS sell (**broken link removed**) and 5% accuracy is about as close as you can get with them, if you want one with a voltage output. With inaccurate sensors, you could run the risk of the fans running and equalising the humidity, but the sensor output never getting close enough to turn the fans off.

If I were doing this, I would have some digital humidity sensors like this:- https://www.digikey.co.uk/product-detail/en/adafruit-industries-llc/3721/1528-2564-ND/8558266, a processor such as an Arduino, a display of each humidity, and a relay output to drive the fans. The Arduino would need a program written for it.

You may then want to look at the costs of just leaving the fans turned on all the time. How big are the fans? I've worked with fans that ranged from about 1 W to 250 kW, and the cost of leaving them running all the time was quite different.

Hey Diver300 - thanks for the reply.
Okay.. wow.. that's might be too much for a beginner?
I understand the concept. I looked up an Arduino (never heard of it) and I understand why you suggested it.

I don't have a size of the fans right now.
I'm trying to move the air around my humidor as there seems to have more humidity on the bottom as opposed to the top.
The dimension of the humidor is Volume = 30×22×29 = 19,140 inches. I believe the CFM would need to be anything above 10.
So a 80MM or 120MM fan would probably work.

So instead of having something that takes into consideration the difference between two humidity sensors. Maybe have a couple fans that independently turn on when the humidity goes above a %. Say like 70%.. I'm trying to equal out the humidity in the humidor so this might work as well. So if I have one fan setup on the bottom and one on the top it should work.

So not to change gears here. But maybe just two fans, with a humidity sensor and a relay to turn it off/on?
Again, totally newbie here! And I have no clue what I'm doing. So any help is really really appreciated!!!

If you are up for a bit of programming that function is doable with an arduino, cheap humidity sensors are available, and there are libraries around to use such sensors such as a Dht22, the bit you'd have to do is fiddle with the readings and decide when to switch on the fans, you can get relay modules to do that task too.

dr pepper said:

If you are up for a bit of programming that function is doable with an arduino, cheap humidity sensors are available, and there are libraries around to use such sensors such as a Dht22, the bit you'd have to do is fiddle with the readings and decide when to switch on the fans, you can get relay modules to do that task too.

Click to expand...

Is there somewhere I can go to get started? I have no experience in building something like this.
Where would I go to begin?

I just googled this:
The arduino doesnt need a programmer and the software (ide) is free to download:
Github has a plethora of libraries for all kinds of things, I reccomend the Dht22 as a humidity sensor, library:
I wouldnt say its really simple, but the arduino is probably the simplest and cheapest way to get into coding.

Very cool!
Thanks!
I do VBA stuff for work.. so I have some familiarity with scripting.
But the building electronics is totally unknown for me.

Codymonster said:

Very cool!
Thanks!
I do VBA stuff for work.. so I have some familiarity with scripting.
But the building electronics is totally unknown for me.

Click to expand...

There really wouldn't be a lot of electronics to build with an Arduino. The programming is probably the most difficult bit for most people but you would be well on the way with VBA.

When connecting up to an Arduino (or really any processor) each function is connected completely separately, and can be checked and debugged completely separately.

For instance, if you connect a fan, I would suggest writing code to turn it on for a few seconds then off for a few seconds and repeating that forever. When you see the fan turn on and off as per your code, you know that bit works.

The Arduino can be powered from a USB wall wart, which is quite convenient.

Agree with diver, the sensors only have 3 wires, and a relay module to control the fan also 3 wires.
There are examples on the net on Dht sensors, and no doubt how to control things like fans.
If your into python some arduino's I believe will run it.

Yeah, it doesn't seem that bad. I've been trying to read up on it.
I think I would need a couple of small breadboards (been looking at seeed too). The Arduino.
Two humidity sensors, however, I've been reading the dht22's are pretty horrible.
So would probably get something else that is more reliable. A relay to turn the fans off/on.
Might as well get a temp sensor too.
I was also thinking it would be nice to have a simple led readout on each fan so I can see the humidity level and temp.
Thinking about having each fan in a spanish cedar box (4''x4''x4'') with slots cut into it for air circulation.
And putting the sensors and a LCD display in the (4''x4''x4'') box as well.
Then the Arduino in a plastic case. Try and make all this look good.

It's not looking too bad. Just the coding to learn.

I think a BME280 sensor will be more reliable than a dht22. I did not reply to your initial post as neither the dht22 or the BME280 would meet your 2% difference specification. (Both of these devices are only specified to +/- 3% so you could get up to 6% difference in readings for the same humidity.) I don't know od any device that would meet your original specification.)

Les.

Les Jones said:

I think a BME280 sensor will be more reliable than a dht22. I did not reply to your initial post as neither the dht22 or the BME280 would meet your 2% difference specification. (Both of these devices are only specified to +/- 3% so you could get up to 6% difference in readings for the same humidity.) I don't know od any device that would meet your original specification.)

Les.

Click to expand...

Eh, it's alright.. I put in the 2% without really knowing the tolerance of the sensors. So I wasn't even aware if my 2% was even realistic.
If 3% is the best I can do then I'll just have to live with it.

Les Jones said:

I think a BME280 sensor will be more reliable than a dht22. I did not reply to your initial post as neither the dht22 or the BME280 would meet your 2% difference specification. (Both of these devices are only specified to +/- 3% so you could get up to 6% difference in readings for the same humidity.) I don't know od any device that would meet your original specification.)

Les.

Click to expand...

I'd suggest the BME280 as well - in my experience the humidity readings on DHT22's seem to differ greatly between different ones, even if placed side by side. The same test with BME280's are pretty well identical.

I metioned the dht for simplicity, I have used a bme280 in my weather station, its more accurate and more reliable, costs a little more.
You also get atmospheric pressure and temp readings with it too.
Some kind of mesh over it would be a good idea, as there a fine hole that can get blocked, I put mine in a pneumatic silencer.