Advice for starting a Project (Heat sensor switch)

[UgeeneCrabs]

Hello, I am in a bit of a pickle but I won't be demanding or expecting answers and the complete solution from strangers.

A bit of a background: I'm designing a project for university, there is two weeks left, and I have minimal circuit designing skills. I have taken a electronics course as part of my curriculum so I know a bit. Therefore I am looking for the simplest viable solution within my capability.

The aim: There are fans to be powered. I want the fans to switch on after a certain temperature (say 70 degrees C). So my idea is to get a temperature sensor (http://www.jaycar.com.au/productView.asp?ID=ZL3336) and implement it in a circuit so that when the temperature is above 70 degrees, the sensor will allow the fans to be powered.

The Question I ask: I don't know if I should use a Arduino or try to make a circuit purely with resistors and what not. I have a hunch after looking at the manual of the temperature sensor it will be complex (I hardly understand it) so perhaps I could use something like this (http://www.jaycar.com.au/productView.asp?ID=XC4230) with an Arduino.

Plan Z (my last resort) would just to build a circuit with the fans and a manual switch to switch it on, and I do hope I can make something basic like that T.T.

Also if anyone knows similar projects or guides can be find it would be greatly helpful. I am going to scour the web now and see what I can find.

Anyways thanks for reading all that, thanks for your precocious time helping a organised noob.

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

You certainly are organised. That's a pretty clear description of your situation!

1. Have you considered buying a temperature controller? Also called a thermostat! Or do you have to do this the hard way?
2. Are the fans mains-powered, or what? You can switch them with a relay, which will work regardless of how the fans are powered, but you may be able to simplify the circuit if the fans are powered from a DC supply that's already available.
3. What DC supplies are available for your circuit to work from?
4. Is any smart control logic required? Or just thermostatic control?
5. What are the physical requirements? For example is there a long distance between any of the parts of the control system?

If you just need thermostatic control, it's pretty easily done with one IC and some passive components. I can draw up a design if you answer those questions. I wouldn't add programmability unless there was a foreseeable reason for it.

 

[UgeeneCrabs]

Hey thank you greatly Kris for the quick (but detailed reply).

1) I've seen thermostats before but they are all the wall mounted ones with displays. Are you suggesting I can use one of them? Or are there simpler, cheaper ones that do the same thing without the display. If so could you give me some key words to use to google? Obviously if there is such a device that can do the task then I would happily use it in the project, the subject I am making this for is a Design subject where we are encouraged to think of solutions that are least technical to solve the scenario.

2) The fans will need to push air about 1.5m. I've looked at the 120mm PC case fans but they aren't as powerful so I am think I will need something different. I do plan using a DC adapter as soon as I find the total requirement of the fans (I plan on using 3). Ideally they the project will be powered from a DC adaptor.

3) I will have to buy a DC adaptor that converts that mains.

4) No logic. We decided that a temperature switch (thermostat control) like I described would be suit our project. If we cannot do that then well fall back to the manual switch (not all will be lost).

5) The fans will be within 30cm of the controller so I think I'll just use some wiring to connect the fans to the circuit.
To give an overall picture. The fans and circuit will all be together in one chassis, out of the chassis will be the DC adapter running to a power point.


Yeah I do agree with the programmability, I'll learn that in a later course.
Hope that makes things clearer, and again I appreciate your help!

 

[KrisBlueNZ]

So are you saying that the fans will be powered from 12V DC?

And what about the distance from the fans (and the controller) to the point where the temperature needs to be sensed?

 

[UgeeneCrabs]

Yeah I looked some fans and DC adapters, they most are 12V.
The chassis (fan and controller) will be ontop of an oil heater (yes, thats our project) so the hot air will waft upwards. The exact temperature isn't critical but the point is to switch the fan on when the oil heater is hot (and thus producing hot air)

 

[KrisBlueNZ]

OK I'll draw something up.

 

[UgeeneCrabs]

Ah thanks a million,
also I think i've found something I could use (http://www.jaycar.com.au/productView.asp?ID=ST3831)
So I could run the "ON signal" thru it and after 50 degrees it will switch on and the fans will run.
I'll wait and see your circuit before I start thinking more than necessary complex solutions :/

 

[KrisBlueNZ]

How about this one: http://www.jaycar.com.au/productView.asp?ID=KC5381

 

[UgeeneCrabs]

I'm not sure which one would be better. If they do the same thing then I'd prefer the cheaper one. I think 50 degrees is suitable but then again it would be nice to have an adjustable one in-case.

 

[KrisBlueNZ]

The adjustable one would be much better. With the fixed one, the OFF temperature isn't even specified. With the adjustable one, you can adjust the ON temperature AND the hysteresis (the distance between the ON temperature and the OFF temperture).

 

[UgeeneCrabs]

Ok I'll probably go with that then.
For the DC power supply, I'm thinking of getting this (http://jaycar.com.au/productView.asp?ID=MP3486&w=12v+adapter&form=KEYWORD) it's 12V 1.5Amps.

The fan's that I'll use will be 12V DC, but the wattage and therefore current of each will vary up to 0.5Amp I found.
I read around that it's ok if the power supply has greater amps than what the system draws because the power supply will just provide what the system draws. So the 3 fans will probably be in parallel so that each get 12V and draw their max current (lower than 0.5amp).
Obviously I don't think the temperature switch will handle the up to 1.5Amps so I'll research abit about relays.

 

[KrisBlueNZ]

Are you sure the fans draw only 0.5A? That's only 6 watts per fan, so they must be pretty small fans.

The temperature controller has a relay on the board. It's the big black thing in the back left corner on the photo. It will handle at least 10 amps.

Yes you're right about the fans and the controller board only drawing the amount of current they need.

 

[UgeeneCrabs]

So do you think the way the temperature controller needs only one input and output directed through the black box, where the current will go directly from the adapter through the relay then to the fans?

Yes PC case fans draw pretty low amps and a 12V 0.5amp would be considered a high powered 120mm fan. I'm trying to find some alternatives but the 120mm size is attractive for the design.

 

[KrisBlueNZ]

The temperature controller needs a power supply - since it's intended for automotive use, I assume this will be 12V DC. It has a 2-pin connector for connection to the temperature sensing device. And it has a 6-pin connector, which I assume is the control output connector and connects to the contacts of the relay.

You would feed your +12V from your power supply into one terminal of that 6-pin connector, where it would go through a contact on that relay and emerge on another pin on that connector. You connect that pin to the positive side of the fans (connect them all in parallel), and you connect the negative side of the fans back to the 0V connection (negative side of the 12V power supply).

So when the temperature controller activates its relay, the circuit is completed and the 12V is applied across the fans.

If you want more information on the temperature controller, try to find the January 2007 edition of Silicon Chip magazine. Or you can buy it online at http://www.siliconchip.com.au/Issue/2007/January.

 

[KrisBlueNZ]

In case you decide to build a controller yourself, here's a design. I've included all of the Jaycar catalogue numbers. You would also need a piece of stripboard, Jaycar catalogue number HP9540.



VR1 sets the temperature at which the fan will turn ON, and VR2 sets the hysteresis (deadband) between that temperature and the temperature at which the fan will turn OFF. RTH1 detects the temperature and should be located on or near the circuit board. Connections are made through CN1 as listed.

 

[UgeeneCrabs]

Ah man you actually found the catalog for the product. I had a look and it helps visualise it much better.
I think I will go with the kit from jaycar but I do appreciate you designing that circuit for me.
I might give it a go in the holidays (when I'll have more time to troubleshoot ect) and perhaps I can use it in my resume (haha even though you designed it).

If you are still willing to answer my continual flow of novice questions,
For the temperature switch kit it needs the 12V supply to power itself, it says "ignition switched power supply and earth" I'm guessing that's the cigarette lighter power supply?
How many amps do you suppose it needs? If its only a few, do you think I could get a heavy duty resistor to connect it with the 12V 1.5A adapter i found earlier to provide a few mA.
I'll have another read over the Silicon Chip article when I get back home later (if you do answer, no need to rush for lil ol me) , I always will be buying the parts at Jaycar on tuesday so I can always ask them.

Anyways appreciate the help so far mate, no doubt i would still be waaay back racking my brains what to do.

Jack.

 

[KrisBlueNZ]

Sure, it's a good idea to go with the kitset.

It's designed to operate from any power supply of around 12V DC. In a car, it needs to be powered after the ignition switch so it doesn't flatten the battery when the car is not running.

It won't need much current. 100 mA (0.1A) should be plenty.

You don't need any resistor between the controller and the power supply. Regardless of how much current the controller draws.

No worries Jack

 

[UgeeneCrabs]

Ah that's a relief then.
But are you saying I can use the same power supply as the one powering the fans?
I was just worried that too much current would flow through the controller (powering the controller itself) because electronic circuits usually have a low current allowance otherwise it would fry.
Because current flows to the path of least resistance, if the internal resistance of the controller is lower than the fans (take the fan as 12V and 0.5A then the resistance works out to be 24ohm) then won't all the current flow through the controller and smaller delicate components?

 

[KrisBlueNZ]

Yes, you can power the controller from the same supply that's powering the fans. Make sure it's a regulated supply, rated for at least 50% more output current than the total that will be drawn by the fans plus the controller (assume 100 mA for the controller).

The controller won't draw more current than it needs. As long as the voltage is right, the controller will be fine.

The internal "resistance" of the controller, between its power supply pins, is a lot higher than the internal resistance of the fans. It will only draw as much current as it needs for its operation.

 

[UgeeneCrabs]

Hey Kris once again thanks for all the help.

This is probably the the finalised circuiting i plan to build. I'd thought I'd run it through you in case of any mistakes.
Below is the attached drawing I made of the circuit.

Hopefully you can read all that crappy handwriting

Also I'd like to ask a few practical questions if you happen to have some know how.
Obviously the relay and and internal power connectors for the themostat have clamp thingys so no soldering required. How would you suppose I connect the 3 way connection between the two fans and the relay output?
My crude solution would be to solder the two fan wires to the wire from the relay output. Also I would be using electrical tape to seal all these solder and exposed wiring so no one gets electrocuted, I'm thinking of using (http://www.jaycar.com.au/productView.asp?ID=NA2829) or what ever jaycart recommends.

Also with the wires, I will ask jaycart for single core ones that can wistand 150 degrees or so (the operating environment is with a oil heater. I might also buy some heat shrink tubing for the wires if they could possibly melt...

Anyways I could go on rambling but I'm sure the people at jaycart will help me with all the physical aspects.

Cheers,
Jack.