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  #21  
Old 07-05-2016, 10:50 PM
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redbeard (Damien)
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Many thanks Chris.

Your Oled display rocks by the way, no PC needed!

Black Sabbath, yeah, got me thinking now... Going to see Iron Maiden on Thurs, Hmmmm

Oops, just noticed, the Mosfets are 20Amp not 40.

Happy to share any other info if need be.

Cheers,

Damien.

Last edited by redbeard; 08-05-2016 at 12:53 AM.
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  #22  
Old 12-05-2016, 05:58 PM
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DavidTrap (David)
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Hi Chris,

Finally got the bits together.

Have managed to finally get an appropriate USB driver for the clone Arduino Nano so I can connect it to my Mac. Sketch has been uploaded and it appears to work! I will have to buy the resistor and sockets and wire them up tomorrow. I'll let you know if the heater gets warm...

Very cool to get this working - feel like a real electronics geek!

DT
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  #23  
Old 12-05-2016, 06:10 PM
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DavidTrap (David)
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Chris,

Can you please clarify what wattage resistor is needed?

4.7k ohm 0.25W, 0.5W or 1W??

Ta
DT
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  #24  
Old 05-07-2016, 08:58 PM
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DavidTrap (David)
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Hi Chris,

Finally reporting back in with the field test of the controller.

The temps at Leyburn were -2 degrees and there was a thick frost on the ground in the morning. The controller was covered in frost where I had it mounted on top of the optical tube. The temperature sensor was a snug fit into one of the collimation bolt holes on the lens cell. The front objective was clear of dew all night. On the occasions I checked the screen, it was running the heater at ~40% to maintain the lens cell 2-3 degrees above the dew point calculated by the unit. I had the dew strap wrapped around the outside of the dew shield - there’s a 1cm gap between the dew shield and lens cell, but it still worked fine.

I highly recommend this unit to others. I'm going to build another unit for a mate with a 14inch OTA - hopefully the mofset plus heatsink will cope with the higher amperage. Better do some calculations.

Thanks again for publishing this design.

DT

Last edited by DavidTrap; 05-07-2016 at 09:02 PM. Reason: added a bit
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  #25  
Old 10-07-2016, 07:38 AM
Garbz (Chris)
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Hey guys, had a chance to lurk today and thought I'd comment on this.

First of nice work looking very good. There seems to be a bit of talk about power draw and MOSFETs so I thought I'd add some advice to the selection:

Firstly you're measuring humidity to calculate dewpoint right at the case of the electronics. If you get hot enough to need a heatsink then this is a really bad MOSFET choice. One-Two degrees of heat passing through the case to your DHT22 will throw out your calculations, and self heating prevents the DHT22 from reading 100% humidity so your dewpoint will follow the temperature and things just don't work (found this out the hard way).

So MOSFET selection. This SHOULD be a non-issue, but the IRF520 is not too good of a choice here. For a dew heater the duty cycles are very slow so a MOSFET will not spend any time in the active region, it will either be on or off. The active region becomes a problem when you're switching in the 100kHz kind of region, but for this application one of the most important numbers is R_ds(on) for the MOSFET.

The IRF520 has a Resistance Drain - Source in On state [R_ds(on)] of 0.27ohm. What this means is that when the power is being applied to the heater the mosfet looks like a 0.27ohm resistor. Drawing 2A through your heater? That's over half a watt for the MOSFET. Thermal resistance Junction to Ambient [R_thJA] for the IRF520 is 62 C/W so the junction of the MOSFET will be 30degrees higher than the ambient temperature. This isn't an issue for the MOSFET, but if you enclose it into a small black box, maybe add a second one for another dew strap you'll quickly turn your box into a heatsource right next to the temperature measurement.

So for ideal component selection rather than looking at the Amps D-S rating, instead look for a low R_ds(on) rating.

If you feel inclined to experiment more you could look at the part I used in my little dew heater controller which was an IR3315 High-Side Switch. This has a couple of awesome advantages over using an N-channel MOSFET:
  • It has an R_ds(on) of 0.02ohm so an order of magnitude better than the IRF520
  • It is high-side switch so it switches off the VCC line rather than switching off the Ground. Using an N-channel FET as a low-side switch is not an issue for a dewstrap ... unless you have a short circuit. On a low side switch there's always 12V on the output, so a short circuit to the telescope itself (most likely place, and usually grounded) is a short circuit even when your circuit is off.
  • Speaking of short circuits using a single resistor the IR3315 can be configured for current limiting, so a short circuit won't damage anything even when you crank the dewheater to 11

Just some free tips . It's a very nice and simple build all the same so thanks for sharing it with everyone.

(edit: Poo! I just forgot one very important detail. The IR3315 being a high side device needs 12V to turn it on. I installed a MOSFET driver before it, a TC4426. That complicates things.)


Also David for resistor wattage it's a simple calculation:
Volts (V) = Amps (I) * Resistance (R).
Power (P) = Volts (V) * Amps (I)

Put them together:
P = V^2 / R.
P = (5*5) / 4700
P = 0.005W so a resistor rated to 0.25W is more than adequate, and as a general rule of thumb typically every time you see a resistor in any kind of digital signal (e.g. 5V output of an Arduino) vs power (+12V side) service it's almost universally okay to select the lowest power rating.

Last edited by Garbz; 10-07-2016 at 08:04 AM.
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  #26  
Old 16-07-2016, 06:30 PM
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ChrisV (Chris)
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Been running mine now for a few months. Some cold dewy nights running at 50-70% power. The mosfet doesnt get very hot.

Under these conditions I've measured surrounding ambient temp - is the same as the reading i get with the sensor.

No probs.

Cheers Chris
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  #27  
Old 22-07-2016, 04:03 PM
redtail (Kenny)
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Hi Chris
I noticed your sketch for the nano dew heater has a button and a potentiometer but the wiring diagram does not show it.
Can you help me out with a revised diagram.
Regards
Kenny
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  #28  
Old 22-07-2016, 04:26 PM
redtail (Kenny)
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Sorry loaded wrong sketch
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  #29  
Old 26-07-2016, 04:35 PM
julianh72 (Julian)
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Congratulations to ChrisV on a nice project!

I hope nobody is offended if I "hijack" this thread - but I was just perusing the JayCar site (that's what I do when it's cloudy, so there's no point in taking the telescope out!), and I came across this project:

http://www.jaycar.com.au/diy-telescope-dew-heater

The parts list runs to a total of about $90 at JayCar (or less if you already have some of the basic components, or shop on eBay):

XC4520 Temperature and Humidity Module: $9.95
XC4454 LCD and Button Shield: $19.95
XC4410 Duinotech Uno Board: $29.95
RN3440 10K Thermistor: $1.75
RR0596 10K resistors: $0.55
RR0572 1K resistors: $0.55
ZT2468 MOSFET: $5.95
XC4482 Proto Shield: $15.95
WC6028 Plug-socket leads: $5.95
RR3264 5W 39Ohm Resistor: $0.48

I haven't done a detailed comparison against Chris's project, and I don't know whether this is a "fork" of his project (or Bob Stephens' 2010 project which he references), but there might be some handy hints for selecting / designing components and wiring, and / or optimising code, for anybody who is thinking of making something similar.
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  #30  
Old 26-07-2016, 10:34 PM
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wasyoungonce (Brendan)
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This is a good project guys...Following with interest...keep up the good work.
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  #31  
Old 28-07-2016, 11:28 PM
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Julian

Interesting. Different to my build - which I ripped a lot from Bob's. My idea was to try and make an easy build with the nice pre-made modules now available (cheaper on ebay).

Bob's (and mine) use a better temp sensor than the thermistor which isn't as sensitive and can be a bit unstable. But the Jaycar version has some nice ideas.

I'll incorporate some of their ideas when I finally getting around to making mine multichannel.
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  #32  
Old 05-08-2016, 08:48 PM
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Attached is an update to the dew heater - includes instructions and updated code. The updates are:

1. changes to the code as one of the arduino libraries was altered which affected its running (well, crashed it actually).

2. You can now run 0- 3 heaters from it, each with their own sensor. It will automatically detect if the number of sensors connected.
- if so = it will control its heater and display its data.
- if not = it will turn off the drive to its heater and not display its data.
NB- there is a change to the pins connecting sensors and heaters.

3. some changes to components - there were some good suggestions about the MOSFET (eg from Damien/Redbeard & Chris/Garbz)

4. Have included more detailed instructions, including how & what arduino libraries to install

Cheers, Chris
Attached Files
File Type: zip DewHeater Update1 5-Aug-2016.zip (265.2 KB, 85 views)

Last edited by ChrisV; 05-08-2016 at 09:01 PM.
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  #33  
Old 06-08-2016, 06:10 PM
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Another update. This version has some testing sketches, so that you can check that each sensor works before putting it all together. Also instructions on that testing.

The final test sketch lets you run it using the computer as the display. So lets you test everything except the display.
Some might like to modify for different types of display. Redtail found a nice red 16x2 LCD display and he's helping me get it going.
Attached Files
File Type: zip DewHeater Update2 6-Aug-2016.zip (314.7 KB, 105 views)

Last edited by ChrisV; 07-08-2016 at 01:14 PM.
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  #34  
Old 06-08-2016, 11:36 PM
redtail (Kenny)
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I would like to thank Chris for all his hard work and lightning fast response.
Another excellent project.
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  #35  
Old 18-08-2016, 08:50 PM
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Quote:
Originally Posted by ChrisV View Post

Or use the freetronics NDRIVE Damien suggested - that's great and only $4 ! The freetronics site has the wiring for it also.

Just remember to run the 12V and GND from the power input directly to whatever mosfet module you use and also to the arduino (as in the diagrams and pics in my post). The mosfet power doesn't come from the arduino.
Superb project Chris and sorry for this probably stupid noob question!
I have got all the pieces and went for the Freetronics MOSFET a la Damien.
The wiring diagram on the freetronics website has thrown me (I'm not well up in such matters).
The S connection is from the -(ve) from the 12V battery supply, the G connector to the arduino board but the D connection should come from the negative side of the load? What would that be and then where does the +(ve) wire from the battery go??

Sorry for my confused state..
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  #36  
Old 18-08-2016, 11:57 PM
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Freetronics has a diagram
http://www.freetronics.com.au/pages/...e#.V7W-JMuubqA

Take the +12v and gnd directly from where the power comes in to your unit.
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  #37  
Old 19-08-2016, 12:39 AM
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redbeard (Damien)
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Quote:
Originally Posted by StuTodd View Post
Superb project Chris and sorry for this probably stupid noob question!
I have got all the pieces and went for the Freetronics MOSFET a la Damien.
The wiring diagram on the freetronics website has thrown me (I'm not well up in such matters).
The S connection is from the -(ve) from the 12V battery supply, the G connector to the arduino board but the D connection should come from the negative side of the load? What would that be and then where does the +(ve) wire from the battery go??

Sorry for my confused state..
You're pretty much on the money. S connection OK, G connection OK, and the D connection is also OK as you connect it to the negative end of the load. Then all you do is connect the positive side of the load to the positive (+) battery. So it's like the load is in series between positive and the mosfet.

Cheers,

Damien.
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  #38  
Old 19-08-2016, 09:24 AM
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Thanks Damien.
So the load in our example would be the dew band? The D connection goes to the negative of the dew band, positive battery wire to the dew band too, well, the RCA connectors anyway..

Stu
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  #39  
Old 21-08-2016, 05:15 AM
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redbeard (Damien)
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Yep, That's it.

All correct!

Cheers.
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  #40  
Old 26-08-2016, 10:25 AM
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Future opportunities in this space

Just had a look at a new Linux device (a Kickstarter project) https://www.indiegogo.com/projects/o...made-for-iot#/ which opens some interesting opportunities in this space. Towards the end of the article it lists a bunch of third part modules which can be used including Current sensing, temp humidity sensors, display etc. I2C, wifi and other bits already on board.

Not available yet from what I can see but an interesting option for the next iteration of dew controllers (and other smart telescope accessories) .
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