Robert Brown's Ardiuno-based dew heater controller

[Dr_Ju_ju]

or a hotter iron....

[JamesF]

I've completed my first board now.  I had a bit of a pause whilst other stuff was happening, but finished things off over the weekend.  Then I set to with FreeCAD to design a box to put it all in which is printing out at the moment, at which point I can start testing.

The actual dew heater bands might have to be a home-brew design.  I have a design in mind that might use a 3d-printed "spine" to hold nichrome wire.  Bills for the barn conversion have started coming in large and fast so astro toys that I can't make myself are going to have to go on the back-burner for a few months I think.

James

[Dr_Ju_ju]

If you have any Paddock electrical tape, that wrapped in an insulating cloth & suitably terminated, will probably do what your after....

[JamesF]

Well, this has taken a lot longer than I was hoping, mostly because I ran out of steam a bit, partly thanks to the weather being so poor, but also because it took some time to decide how I was going to put it all together.  However, this evening I have finally finished construction of a dew controller and am now in a position to start testing.  It's all in a 3d-printed box because I couldn't find anything with a design I liked in a suitable size, though in fact the box had to go through a few iterations because there's an awful lot of wiring and getting something that wasn't too big that I could still get my fingers inside to connect everything up turned out to be a bit of a challenge.

James

[JamesF]

Well, most of the tests appear to pass.  The temperature sensors all appear to work as does the humidity sensor, the fan comes on when it should and so on.  I do have a couple of failures however.

First is that the RGB LED doesn't do anything when the  test to light it is run (at least, I think that's what the test is supposed to do).  So I need to check that out.  Could be that I have disturbed the connector whilst closing up the case.

Also, the LCD display doesn't actually display anything, though it does light up.  I hope/suspect that is because the brightness needs adjusting.  There's a small PCB-mounted pot on the back of the display for that purpose.

After that I need some dew strips to test the actual heating side.

James

[JamesF]

LCD now works.  It was just a case of adjusting the brightness.

James

[JamesF]

And the RGB led also kind of works now.  Only I have the red and blue connections swapped over.  Fortunately I think that should just be a case of pulling the Dupont pins out of the plug and swapping them.

James

[JamesF]

And the LED is now sorted

All that's left to test is the MOSFET/PWM side of things.  There's what appears to be a MOSFET test with all the other test code, but not much information on how to check it.  Given that it's PWM, I don't know if it's possible to check the output with a standard multimeter.  I don't imagine that any harm can occur if I try it, but I'm not sure I want to assume so.  The same applies with the PWM test.  The code says "You MUST HAVE the dew straps connected for this to work properly".  I'm not sure how you tell if the straps are running at 50% or 100% though.

James

[JamesF]

I ran the MOSFET/PWM tests and the LEDs for each channel show at varying brightnesses for the different heating levels which is a good sign.  I guess the final test is to actually plug in some dew straps and check they get warm.  I have some old ones that I can test with if I can find them in the observatory so I'll do that tomorrow perhaps.

Initial thoughts now I'm nearing the end for this first one are that I'm really quite impressed.  It's not a hard build and looks like it should do the job well.  Probably the most awkward part is fitting it all in a case of a reasonable size.  There are eighteen components that don't go on the PCB in mine, and I even left out the "give it max welly" switches for the heater strips.  That's an awful lot of wires and means there's a bit of a balance to find between the placement of all the plugs and sockets and the sizes of the case needed to fit them in.  Putting most of them on the top of the case makes for much easier fitting in a smaller case, particularly for someone like me who doesn't possess the most delicate of digits, but it does mean wires end up spewing out of the top of the case once everything is connected.

I'm not that happy with the LED bezels.  I bought from several different Amazon/Ebay vendors and they're really all pretty skanky.  Whilst I did use them in the end, I also applied a bit of hot-melt glue to the inside to keep them in place properly.  That's no fault of the controller design though, just cheaply-manufactured plastic tat.

There are probably three awkward bits.  The first is down to my decision to fit pin headers for all the off-board components.  The pin headers are nice and easy to fit, but then I ended up making a loads of connectors to go on them, crimping the sockets onto individual wires.  That was exceptionally tedious.

The second is down to the use of 6P4C/RJ11 connectors for the temperature sensors.  This isn't of itself a bad idea because lots of 1-wire kit uses RJ11 or RJ45 plugs which means the sensors can be used elsewhere if the "standard" pinout is retained.  The problem really comes down to not being able to find any reasonably-priced way to connect wires to the sockets.  It's possible to buy little break-out boards for the sockets with solder pads to connect wires onto, but they're over £6 each and I wanted a total of nine for the three controllers I intend to build which ramps up the cost a bit.  There are some available with screw terminals, but they're huge.  At one point I did seriously consider getting the gear to make my own PCBs and do them myself.  It would probably pay back when it comes to making the PCBs for the focuser units that I intend to build once the dew controllers are done.  In the end though I just soldered the wires direct to the pins of the RJ11 sockets and sleeved them with heatshink.  Not particularly neat given the pin spacing and the fact that my eyesight isn't quite what it used to be.  I don't know if anyone sells stripboard with the correct spacing (1/20th inch?  I'm not sure it exists), but I'd be tempted to use that next time unless I can find some with, say, a punchdown style connector that's sufficiently small.

The RJ11 sockets are actually quite awkward to fix in place in the box too.  Because mine was 3d printed I ended up making the openings very close to an interference fit and then getting out the hot-melt again

The final thing which I think I've mentioned in an eariler post is that some of the diode leads don't fit the standard hole size in the PCB.  I bodged it for this build, but for the next one I might well take leads off the board to a terminal block and fit the diodes there.

The only teensy-weensy little criticism I might possibly have of the project is that the documentation is a bit haphazard and I'm not sure all of it is in sync with the different versions of the design.  Patience gets you there though.  It's not a huge deal.

Now I have a better feel for what I'm doing I might well record the next build and do a step-by-step write-up to help anyone else who wants to give it a go (as well as making my case design available).  I'm sure anyone who can wield a soldering iron accurately (not a lot different from being able to colour in between the lines, really) could manage it.  I'll admit to always having had an interest in electronics and having an OA Level in the subject, but I'm not an electronic engineer and haven't spent my life building this sort of stuff.  In fact I've often bemoaned the fact that I don't really understand enough and it's hard to find somewhere to start that really gives one a good grounding in electronics design for the interested amateur.

James

[JamesF]

Tonight it was vaguely clear at times, so I took the box of tricks out to the observatory and connected everything up to my 80ED and ST102 (guidescope).  I had one little niggle as a result of the switch connection coming loose internally, but that was probably me not making it well enough first time around.  Otherwise everything seemed to work as expected.

After four hours on a very dewy night, with dew actually dripping off other OTAs, the optics on both of the protected scopes were perfectly clear.  I am very happy about this

There are a couple of things I think I still need to sort out.  First the reading of ambient temperature is a little high.  All the temperature sensors I have in my Stevenson screen were reading below 5C when the dew controller was reading 6.8C.  I assume that's just an offset somewhere that I need to change.

And second I think I need some what to insulate the temperature sensors from the outside world so they're not losing heat and mis-measuring the temperature of the optics.  I suspect a bit of camping mat will do the job just fine.

James