late enthusiast...

Hello everyone,
in my late 30`s electronics became my hobby... I`m an chem engineer by profession and want to combine it with electricity manipulation knowledge. I´m going to read and study...but I prefer to learn from examples. I have the following problem: constructing a climate control chamber for my experiments: T and rel.hum. control I can manage (comparator, transistors, 555, ...-not so hard to understand - simple on/off control) but I want this addition: I have 3, 3-pin 12V/DC fans, together drawing cca 0,9A (starting voltage ~7V). They are going to control access humidity via sensor (the one with linear voltage output, supply 4-5,8v/DC). In my desired range (45%-80%) voltage rises from 2,2 - 3,2V/DC respectively. System is going to be powered from 12V dc adapter (1,5A max). I want my analog circuit to do following: turn ON the fans at set values (for example 55%, 65%, 80%,....) on full rotation speed and with decreasing humidity gradually slowing them down to standstill - that is 5% rel.hum. bellow the set value (80-75%).
Can someone please advise me the smoothest way to accomplish that.
Thanks.
Matej

Hi and welcome to the ETO forums!

Likely, the best place to start is at the beginning. That would be the choice of sensors used to sense temperature and RH. The sensor outputs then will likely need some form of signal conditioning. If you have a link to the humidity sensor data sheet that would help. Something like this can also be built around a PIC chip or just using discreet components depending on how complex you want to get. Anyway, the sensor signals would end up likely going into a comparator circuit or two and the output of which would drive the fans or other hardware. Now if you also want fan speed control then we could add maybe some PWM (Pulse Width Modulation) to control the fan speed depending on the RH. Temperature could for example use a little LM35 as a sensor, did you plan to control temperature?

Anyway it would begin with the sensors in my opinion and work from there.

Ron

Hey Ron, thanks for the swift reply. Yes I will control the temo - simple hysteresis with the comparator (negative resistance thermocouple) to keep it +/-2°C at set value. The humidity sensor I had in mind is HIH4000 (Honeywell) which is a capacitance sensor with an add-on so it has voltage output linear to humidity.

Before I forget, this is good reading on voltage comparator circuits. The choice of humidity sensor should be fine and can be coupled directly to a comparator. Since you have three fans it would be easiest obviously to turn them on in sequence based on RH. Not to say running them in PWM would make things all that much difficult but if you could live with the three fans turned full on sequentially based on RH it would be simpler. Don't know the temperature range? When you mention:

Yes I will control the temo - simple hysteresis with the comparator (negative resistance thermocouple) to keep it +/-2°C at set value.

Click to expand...

I assume you mean a thermistor with a negative temperature coefficient?

Ron

Sorry but my terminology is also at the beginning yes I meant that NTC thing... My desired ranges are around 20 and 40 °C and control the relay that runs 220V/AC heater. I don`t see any problem putting together a simple circuit to do that. The fans are little more of a problem - one is outtake , other two are intake each covering different zone with the same T and hum. conditions - so I need to control the speed. I spent some time reading posts on this forum and found that PWM control using 555 would (maybe) be the best alternative. The fans are 3-wire with possibility to use 4-wire adapter - does this mean they can be PWM controlled? Could you direct me to some schematic that would roughly do that? Thanks.

Matej

Before I forget, a three wire fan is just a fan with a tachometer out signal on most small 12 volt fans like those used in home computers. You are really only using 2 of 3 wires unless you want the tach signal out for some reason? A small overview of fan speed control can be found here. While it does not show detailed circuits to create PWN it gets the idea across. I believe what you really want is fan speed control that will be controlled by humidity? I think what you want would be a voltage controlled PWM circuit. Where as the humidy sensor outputs a voltage we use that voltage to change a PWM signal that in turn controls fan speed. Rather than a 555 based circuit I would think about maybe a circuit along these lines using a comparator. The comparator will be your new best friend. I would not use the LM741 shown in the circuit but a better comparator and let that drive a transistor to drive the fan(s).

If I have time tomorrow I'll try and get a rough circuit diagram for you unless another member has something to add? More suggestions are welcome.

Ron

Thanks Ron. I think I`m getting closer to my goal...the simplest solution is always (almost) the best solution in my world. When I get to understand the comparators and the accompanying circuitry and off course the real nature of transistor I`ll be off ... maybe to the digital world and software; if you find time for that example circuit I`ll much appreciate it.

Matej

Attached is an example of what I was getting at for PWM using a comparator. A rough example made in haste but an example. Keep in mind, this is likely how I would approach it sans just buying equipment.

What we are doing in the attached is simulating an input from a triangle waveform generator into a comparator. The red trace simulates the triangular waveform. It is labeled as V2 in the schematic. It is about a 500 Hz. triangular waveform. For real PWM we would likely go higher in frequency. That signal goes into the - (Inverting) input of the comparator.

Next we have a potentiometer labeled U2. This pot simulates the output of the RH sensor. However, no, it is not scaled and signal conditioned. Just as an example. This would be the V Reference fed into the + (Non Inverting) input of the comparator. That voltage level will vary based on RH.

When the level of the triangle wave is below the value of RH the comparator output the comparator output goes High turning on transistor Q1. That places the collector of Q1 at just about 0 volts. When Q1 is off the collector is high and when Q1 is on the collector is low. This can be seen on the Vpwm signal.

A typical 12 VDC computer fan draws <200 mA as I recall. The 2N2222 can handle about 800 mA so as shown the 2N2222 could likely drive 3 fans with a heat sink on it. Resistor R4 is just there to simulate a fan.

Again, short of using a costly RH controller I would likely go along these lines. Not to say this is the best way to do it, just how I would approach doing it. If any other members have suggestions I would love to hear them.

Earlier I posted a link to a comparator website. You should see a connection between those pages and what we have here.

Ron

Thank you. It is clear - needs the finesse from my part and I`l let you know the resulting ""... The fans are 10 inch in diameter and draw 330mA each...
Have a nice day!
Matej

OK, then a larger transistor or I would likely use a logic level mosfet to drive the fans. Let me know how things are going with the project.

Ron

Halo again!
Had lots of "other" work, today I found some time to draw this. What do you think - is it going to work? The pot on the left is to set the reference voltage (1-4V) and the oscillating voltage up and down this value is +/-0,15V (5% rel.hum.) determined by resistors. The frequency should be cca. 500 Hz. By "work" I mean - I set the Vref so that it corresponds to Vout from humidity sensor. Will this hold the moisture in +/- 5% of the set value? I mean roughly off course
I did this in a hurry so don`t joke if I`m completely off the course...
Thanks.
Matej

ps. don`t mind the comparator type - I plan to use lm339 or lm324. And the thing down is the 5V reference...
and another thing - are the inverting/non inverting connections OK - so that the fan sped increases with raising humidity (raising voltage of the sensor).
...again...maybe I should use PNP? frequency? and use pot (3k or 5k) instead of 1,33k resistor to set the desired "bandwidth" ?
oh...and the sensor is also grounded and it is not +/- 0,15V - as it is drawn it is +/- 0,08V...and the capacitors on 5V reference are wrongly connected...

I will look at it tomorrow evening and try to run a simulation on it. Didn't want to leave you hanging.

Ron

The attached circuit is along the lines of what you want. It is far from a good working design but gets things across. I used the LM741 simply because it was available, I would actually use another op amp like a TL084 likely. In the level shifter I use it to shift level and attenuate and using an op amp as an attenuator as I did is not a great way to do it as it can cause stability issues with some op amps. This entire affair was done in haste.

The idea is to get a triangle wave between 1.5 and 3.5 volts pk. to pk. that the relative humidity signal is compared to. Over time I show the RH increasing and as it does the PWM signal follows it.

Have you run the circuit you made in a simulation? I had problems with it and understanding it?

Ron

Your pot for the reference voltage does not have its slider connected so it is simply a fixed 50k resistor.
Your opamp U3 has no negative feedback so its output will be saturated high or low depending in its input offset error.

The circuit from Reloadron has a dual-polarity supply.

I redraw the schematic - my idea is this: The humidity dependent sensor produces voltage in range 1,5 - 3,5V. I`ll use 2,5 - 3,2 V (50-80%rel. hum. - with the pot on the left I should set the divider so that Vref is equal to desired humidity (desired sensor output voltage). The amplitude of the triangular wave is determined with R4 and R5 (Vref+/-(Vref*R4/2R5)) - one of those resistors is variable, lets say R4; frequency is determined by R3, R4, R5 and C1 (f=R5/(4C1R3R4) and it ranges from cca 120 to 220 Hz (depending on R4 setting which is between 5 and 10k, depending on the Vref chosen). The result should be - if the sensor voltage goes above the triangular wave - the fans are on 100%, when it reaches the top of the wave it starts to reduce speed and the fans come to standstill at the bottom of the wave - 5% difference in humidity...and other way around - when humidity rises to the bottom of the wave the fans start - jump start capacitor (C6) and later the speed adjusts to the wave. The aplitude of the triangular wave must be equal to 5% change in humidity = 0,15V. Matej

I tried reconstructing your circuit and running a simulation. I did tie the R4 wiper off. I also am not sure what U4 is doing? I see you are using LtSpice, have you tried running a simulation of the circuit?

Ron

Yes, I`m trying to use LTSpice...started week ago - drawing is so so, but simulation is another thing-still struggling with it;( U4 is the fourth op_amp (LM 324) and so that it is not just left doing nothing I "tried" to connect it in the way that it acts as voltage follower and buffers the sensor output...maybe someone could explain this to me too. I`m grateful for all your suggestions. Oh...and the pot + resistor that simulate humidity sensor are not cool, the pot should be at the bottom...I think...how did you come to the component that you used in your sketch to simulate the sensor with voltage output? If I try simulation I just get error messages - can`t set the parameters, can`t define the input/outputs....
As I was looking at some articles and did additional calculations I noticed that I can not get the symmetrical wave +/- Vref unless Vref=Vcc/2 - so maybe could you explain Ron, is the part at your circuit named "level shifter" supposed to toggle with the triangle wave positioning so that Vref (middle of the wave) can be made equal to desired voltage output of the sensor?

Matej

now after couple of hours I finally managed to get a grip on simulation - it`s much easier now...I`ll post my final circuit soon and wait for your review

OK and feel free to modify and play around with what I have posted. Actually I am terrible with LtSpice but stumble along with the simpler things.

Ron

I have finally accomplished what I wanted - now the finesse and stability... I`m quite proud of myself - very little time and still fast Look at the pictures and say your opinion... R4 and R6 (or R7) are going to be variable or I`l put fixed resistor + small pot to fine-tune the thing.

Matej

Excellent, very good job. Pretty cool when it works. In time as you do more drawings you don't really need to run your power lines all over. Just use nodes like you see in my cartoons.

Nice job again....

Ron