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kbrown

Arduino Controlled Scope Fan

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Hi, 

I've been working on a DIY fan for my 10" Skywatcher newt. My plan is to measure the ambient temperature and the mirror temperature with two separate thermistors and control the fan speed depending on the temperature difference. 

My question is, does it make sense to use the fan if for some reason the ambient temperature is higher than the mirror? The fan is sucking the air out the tube at the moment. 

IMG_20180220_211331.jpg

IMG_20180220_211415.jpg

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I think it makes sense - in my opinion what you want to achieve is to equalize mirror and ambient temperature regardless what is the difference. But when mirror is colder than ambient then moisture may condense on the mirror. 

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I agree with @drjolo comments. The idea is to bring the mirror to ambient temperature as quickly as possible. Measuring ambient temperature is easy. Mirror temp may be more difficult as glass (if a glass mirror) is not a good heat conductor. Therefore cooling (or possibly warming) the outside of the mirror quickly may result in the inside of the mirror being at a different temperature. Still, I understand the overall aim is to reduce convection currents in the tube due to temperature differences, so it is the temperature of the outside surface of the mirror that is important. Perhaps a solution would be to keep the fan going for a period after the mirror and ambient temperature appear equal. No need for fan speed control perhaps - just on or off.

HTH

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Thanks guys. Your comments confirm what I was thinking as well!

Speed control is not really necessary but I'll throw it in just to get a feel for it. I already have a tach signal from the fan and pwm input on it to control the speed. Both easy to hook up with the arduino. There's an easy to use PID controller library available too so most of the heavy lifting is already done. I think ?

Edited by kbrown
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Just got this thing in a usable shape. Mechanically and electronically I think it's pretty much done. The software running on the Arduino is preliminary but it'll do for now. It measures ambient and mirror temperatures, supply voltage and fan speed. These are printed out on the serial port for testing purposes. It won't try to run the fan if the input voltage is below 10V.

The fan will run 100% speed if the temperatures differ more than 4C, 75% speed at 3-4C difference, 50% speed at 2-3C and 25% speed at 1-2C. Below 1C difference it won't bother to run the fan at all. I've just guesstimated these values as I haven't been able to test it in the field yet.

Here's some pics of the fan assembly:

 

IMG_20180415_233649.jpg

IMG_20180415_233838.jpg

IMG_20180411_225350.jpg

IMG_20180411_011256.jpg

IMG_20180411_011248.jpg

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Very neat assembly! Now the tricky bit - quantifying how much of a difference the fan speed makes. :happy11: Cool down (up?) graphs would be interesting to see.

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1 minute ago, bobro said:

Very neat assembly! Now the tricky bit - quantifying how much of a difference the fan speed makes. :happy11: Cool down (up?) graphs would be interesting to see.

Thank you, sir! Definitely something for the future to test. Wouldn't be hard to print the data in csv format and do some graphs. For now I just wanted to get this in some sort of usable state so I can take it with me on my upcoming holiday in few weeks time.

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Very nicely made! Just out of interest, where did you get your PCB manufactured? I'm looking for cheap options for my Arduino SQM - either that or make my own CNC machine.

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2 hours ago, CrashEd said:

Very nicely made! Just out of interest, where did you get your PCB manufactured? I'm looking for cheap options for my Arduino SQM - either that or make my own CNC machine.

I made it myself. You don't need a CNC machine for that but you do need some chemicals and a few supplies plus practise to get the process working reliably. There are several methods but the one I'm describing here works the best for my purposes. It can be messy and might not work every time but for one-offs it's quicker and cheaper than to get them done somewhere. Worth it if you do it frequently enough like I do. I do have a CNC router but I don't use it for engraving the PCB traces. I do use it to cut the PCB into shape and drill the holes which you can do without a CNC machine too if your design is simple enough.

I use pre-prepared photoresist pcb laminates if possible as they're easier and quicker to deal with. Sometimes if the development goes wrong and I have to strip the photoresist layer off to start over then I use Photoresist Dry Film to get a new layer on. These are usually positive photoresists as opposed to the original negative photoresist layer so to develop it you'll need print your design inverted. Forget the photoresist sprays as it's really hard to get an even and dust free coat with them.

I print my designs using a regular laser printer (set to darkest possible setting) on LaserStar PCB printing film if it has really fine lines and narrow gaps in it. If it's a simpler design with thicker lines then the cheaper laser printable tracing paper will do the trick.

I use an old Phillips facial personal solarium (cheap second hand from ebay) with 4x UV lamps to expose the pcb sandwiched between two panes of window glass.

After the UV exposure I develop it using universal PCB developer solution (SENO 4007)

After successful development I etch the PCB in a self made etching tank which I made out of a plastic jug, a fish tank warmer and a fish tank aeration pump. So far I've been using ferric chloride to do the etching but it's quite messy and does age and loose its power over time. Once the solution I currently have has expired I'll be switching to another type of etching solution made of hydrochloric acid and hydrogen peroxide which will last longer and from what I've read should actually get better over time the more copper gets dissolved into it.

After etching I usually immediately tin the bare copper (before it starts oxidising) using Seno Immerse Tinning method which is quite simple to use. It'll give extra protection to the copper layer and also makes it easier to solder.

Another useful thing to do is to either add a solder mask (I've used Dynamask 5000 in the past) or use Green Coat Solder Lacquer like I did in this case. The lacquer is easier and quite durable if you follow the instructions. Either way this will protect the PCB even more and make the soldering process easier.

Sometimes I want to get the component layout and annotation on the top side of the PCB. For this I've successfully used the photoresist dry film mentioned above. After developing and curing the film it's a good idea to protect it with some clear lacquer.

Happy to shed more light on the details if you wish?

Edited by kbrown
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Ran into an interesting issue with this setup. I had my scope out for quite some time and the mirror temperature was pretty much the same as the ambient so the fan didn't run. However after a while my main mirror started dewing up which it has never done before without the fan setup. I'm guessing it's reducing or preventing the air to circulate as usual if the fan isn't running. Guess I should have it running slowly all the time...

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