Article: Dew Heater Control and the Arduino [Archive]

hikerbob

07-07-2010, 04:37 PM

Mike thanks for publishing that. Hopefully it will provide a useful starting place for others wanting a dew heater controller. The design should be able to be enhanced or dumbed down (I use a more basic version to provide multi channel dimming of Led lighting in my camper trailer).

I found an additional resource today which could be useful for anyone looking a using the idea's here but is able and willing to do some alterations. A Serial LCD controller based around an Atmel chip ( http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1260399444 ). I've not worked through what's involved yet but it could be useful.

wasyoungonce, thanks for your comments. Please feel free to float any improvements you can think of, I've been learning on the job with this. I'm very happy with the outcome but suspect that there will be a range of other options.

Bob

wasyoungonce

07-07-2010, 04:58 PM

Excellent stuff Bob. The only thing I'd look at is maybe higher pwr Fets. I used IRF4095 "p" mosfets, $1.44 ea, capable, ...well in my design ...I've run at 4.8 amps per channel but recommend 3A per channel (with a simple Heatsink). I mean who would need more than 3A per channel I ask? :eyepop:
But that would mean a complete re-work and don't I know it...you have to settle on a design and stop changing it at some stage!

I am trying to work out the STA508A ATM...just a little confused by it. Its a H bridge driver?

But using an Arduino. Yep that's the ticket. Great work putting it on eagle PCB. I did as well.

Well done indeed!:thumbsup:

edit:

the more I see of it the more I like!

wasyoungonce

07-07-2010, 05:13 PM

I meant to say as well. After I pretty much finished my designs I ordered a Duemilanove kit (on it's way as we speak) as I was going to play with 6 channel PWM after re-reading your posts from last year.

Sigh...beaten to the punch!;)

hikerbob

07-07-2010, 06:18 PM

wasyoungonce, I've been pondering a rework of the design to use individual fets just because the STA509A is getting hard to get.

The STA509A made a lot of sense on veroboard, not so much on a made up PCB. The STA509A is just 4 Fets with the source's already joined together which works well for the way I'm using them (and trying to do the switching between the 12V supply and the heaters looked difficult).
I won't be embarking on those mod's at the moment as I'm hoping that discussion will attract some feedback on power stage filtering, it's not been a bother to me but for those doing serious photographic work it may be an issue.

If it helps the amperage of dew-not heaters is listed on their website http://www.dew-not.com/specifications.htm I don't know what other straps draw.

Bob

hikerbob

07-07-2010, 06:21 PM

I've just spotted the typo which identified the the fet array as STA508A rather than STA509A - sorry about that.

wasyoungonce

07-07-2010, 07:23 PM

Yes I am pretty much facing a wall with power filtering. Problem is switching Hight current effects everything attached to that line....well it does on my power supply. If I had 2 ccts at 3A and another 2 and .5A...a heap of switching inrush!

I have some high current chokes from jaycar I was going to put in line & fiddle...some day!

Iceman is going to put my ccts on so you can see where I'm at, unless you want me to send you my eagle files now?

Another issue is FET on rds. I pretty much faced pdip (package power dissipation) problems when trying many FETs with .117 milli ohms rds (or so) at 3.5A. Each TO220 FET package can dissipate around 2.5W (Max) and rds on rises with I draw & pdip case increase.

So I turned to very low on rds fets....020milli ohms (IRF4905)! My ccts were designed with these low rds FETs..less pdip issues. But ATM but I am greedily eyeing some new generation automotive intelligent power switch FETs that have I fold-back... like the IR1334 (https://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=IR3314) or IR3316. They are designed to run from 12V and have current fold back by using a resistor and/or even monitoring with something like an AVR.

I looked up commercial dew heaters & settled on a 3A per stage max output (with a little head room). Any more is for the big boys!

But I got to say I love your work.

Omaroo

08-07-2010, 08:57 AM

Thanks Bob! I need a little project in my life, and although I build my own PWM dew controllers already I'd like to investigate this one. Looks very interesting. I'll start investigating parts today. :thumbsup:

troypiggo

09-07-2010, 04:18 AM

Thanks for posting and taking the time.

Minor point - the links in the article to the Arduino.cc website seem to be duds or not quite correct. I didn't check the others, but thought you might like to know.

hikerbob

09-07-2010, 07:33 AM

Troy thanks, I'd not rechecked links recently.

The link to the Arduino hardware page is http://arduino.cc/en/Main/Hardware

That lists all the official variants, there are a variety of unofficial ones as well such as the Sanguino, Freedunio etc.

Bob

hikerbob

09-07-2010, 07:44 AM

I'd not completed a PCB version when I originally submitted the article and somehow in the revision process the photo of that got missed.

hikerbob

09-07-2010, 12:19 PM

I received some LCD displays from Satistronics today. The backlight does not appear to be working on any of them. I need to do some more investigation and I've emailed them to advise of the problem and to seek advice.

They appear to be identical to a number of display's I already had which don't have that problem (using the same dew controller and Serial LCD backpack).

I'd suggest anyone thinking of ordering them hold off until I see what their response is.

I also got some headers and pin strips from them which I use to connect the LCD, arduino and serial LCD driver to the board and they are excellent value compared to what I pay locally.

AU$32.32
1 of. FEMALE HEADER PITCH 2.54mm V/T TYPE 1x40 80pcs/panel (http://www.satistronics.com/female-header-pitch-254mm-vt-type-1x40-80pcspanel_p1983.html) AU$8.83 (not available at Jaycar but from recollection generally about $2 each from Prime)
1 of. PIN HEADER PITCH 2.54mm V/T TYPE 1x40 200pcs/bag (http://www.satistronics.com/pin-header-pitch-254mm-vt-type-1x40-200pcsbag_p1991.html) AU$12.47(75c each from Jaycar for larger numbers or 95c each buying them individually)

It should be a long time before I have to buy either again.

hikerbob

14-07-2010, 09:28 PM

I've discovered that the backlight connections are reversed on the LCD displays. I've still not received a response to my earlier message to them, I've sent on the additional information.

Bob

tjsteele

06-05-2011, 04:20 AM

I was looking to build my own dew heater controller and found a couple different designs on the internet. One had a temperature controller and wanted to combine it with another pulse modulated project using a 555 timer. Then i found your post. This is exactly what i was looking for...

Has there been any modifications to this plan since it was posted (back in 2006) or has anyone else contributed to the design? For some reason i was expecting a very high cost but after looking at the parts, i find that the unit can be build for pretty cheap and will start looking into it very seriously. Can i still contact you guys for assistance if i get lost in the process (which i'm sure will happen - never built anything electronics in 20 years)?

I'm also new to astronomy so after trying my telescope a couple times last fall, i realized that this was needed badly so started looking right away. If i can build it, i would rather do that. Plus that's more fun to say that i built it then to say i bought it...

Thanks, T-J

hikerbob

06-05-2011, 06:59 AM

John are you on the right thread? My article only went up last year, not 2006.

Happy to help where I can. I need to get back to it very soon and finish another controller for a friend.

Bob

tjsteele

06-05-2011, 07:18 AM

My bad. Been following too many links :eyepop:. But at least i got the right link for your build, and that's the project i need...

This adruino stuff is all new to me but tha's the way to go for sure. I studied electronic engineering back in 86 and i just wish i would have kept working in that even as a hobby but life took me in another direction. I'm sure i'm gonna pop a few neurons in this project but it's worth it...

hikerbob

06-05-2011, 09:15 AM

No major updates at the moment then.

There is also some work being done on Ascom control at http://tech.groups.yahoo.com/group/sgl_observatory_automation/

I'm not sure where things are up to with the dew heater stuff but the group was heading in a similar direction (except with Ascom control). I'll revisit that at some point, at the moment the controller run's standalone but it would be nice to have a GUI interface to tweak settings on the fly.

I have some partly made up boards sitting around (still needing some components ). if you wanted to have a play with one I could probably send it over. As they say on the boxes though - some assembly required.

WasYoungOnce also did some work on a dew heater which looked good.

Feedback from mine - the low side switching leaves the outer shroud of the heater connector's at 12Vdc. Some risk there of an accidental short to ground. I've played with other housings for some camping lighting using a 240V moulded box designed for weatherproof mounting of a power point or switch, it had a double wall on the sides which allowed the RCA sockets to be recessed. I've not tried to see how my board would fit in one yet.
In some way's I've wondered if I would have been better off incorporating the control gear into a larger power/connection box which sit's on the ground and running the cables to the scope as a loom. At the moment it's more junk hanging off the telescope with loops of excess cable etc. I may may redo things to do that in the near future.

Bob

tjsteele

07-05-2011, 04:54 AM

For me, the case and mounting locations would be the least of my worries. Being in Canada, the dew is pretty intense so having a case on the ground is out of the question. I would want a smaller case and probably mount it on the telescope mount using a secondary bracket. Don't want it on the telescope itself either (by the way, I have a Celestron C6-SGT, nothing fancy but it's my first).

As for the temperature Sensor (http://www.google.ca/imgres?imgurl=http://img.tomshardware.com/us/2005/09/23/paying_the_price_for_high/temperature_sensor.jpg&imgrefurl=http://www.tomshardware.com/reviews/paying-price-high,1129-2.html&usg=__wRB2wGBZpEUFemLDbwzPTk-P_vU=&h=257&w=425&sz=13&hl=en&start=121&zoom=1&tbnid=F4Hyw1cTH--svM:&tbnh=97&tbnw=161&ei=aDTETc_PBInu0gHtuaihCA&prev=/search%3Fq%3Dasus%2Btemperature%2Bs ensors%26um%3D1%26hl%3Den%26client% 3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26biw%3D1021%26bih%3D70 9%26tbm%3Disch0%2C3306&um=1&itbs=1&iact=hc&vpx=699&vpy=376&dur=321&hovh=118&hovw=196&tx=189&ty=70&page=7&ndsp=21&ved=1t:429,r:4,s:121&biw=1021&bih=709), i was thinking of using computer temperature sensors such as the one that comes with Asus boards. They are tiny flat ribbon like sensors and could be mounted anywhere on the telescope permanently if it works.

And you would send me a board? I would love that, so just tell me how much and i can do a Paypal thing or whatever you prefer, but as i mentioned, i'm in Canada. I already located most of the parts for the project and was gonna order asap. Spent the better part of the day looking at your diagrams as well. Man my theory is so far gone it's not even funny. But just to let you know, I work in a University so i have access to some pretty knowledgeable people as well if i need assistance...

I did look at the exchanges you had with wasyoungonce as well. I think this is where i got confused with the 2006 thing. If not then i'm totally out of it.

hikerbob

07-05-2011, 09:39 AM

John PM me your details and I'll find out what the postage will cost.

Bob

DancesWithWords

08-07-2011, 09:10 AM

Bob,

Do you have a complete list of parts. I want build your controller. Thanks.

==
Bruce
Member: SPIE, IEEE, IACR
Faintfuzzies.ca

hikerbob

09-07-2011, 08:24 AM

Bruce the list in the article is pretty close

http://www.iceinspace.com.au/63-597-0-0-1-0.html

I'm working towards a rebuild and going with a larger housing and stick the internals of a USB hub (7 port) inside (with 5 ports facing out) and additional power sockets.

Feel free to ask if you have additional questions.

Bob

DancesWithWords

09-07-2011, 08:47 AM

Did I read that in your new controller you are using the Arduino Mega? Do the Arduino have the capacity to record temperatures collect from sensors please on mirrors and a remote sensor for ambient temperature. It be really groovy if that info could then be displayed on the LCD/LED screen to see how close the scope is to equilibrium. Bonus points if the Arduino had memory capacity to store this info for later retrieval via USB or bluetooth.

It this project possible for someone with no electronics background? Thanks to my wife, I now have a New Hakko 936 Soldering Iron, and a Extech 300 Multimeter. Birthday presents. :D

Oh, I've got a Orion XXT12 w/ the Intellicontroller thingie (http://faintfuzzies.ca/v/hobbies/astro/truss-dob/DSC07119.JPG.html). I'm in Canada and the LCD is the pits in the cold. Anything under 10C and it falls to function. Do you know what the tolerances are on the screen you are using is?

Maybe more questions later.:thanx:

==
Bruce

DancesWithWords

09-07-2011, 09:35 AM

What are you going to use the addition USB ports for?

==
Bruce

hikerbob

10-07-2011, 09:00 AM

Bruce I'm trying to cut down on the number of extra things that hang off the scope. The USB ports are for camera's, focussers, Bluetooth dongle for EQMod and a wireless receiver for a gamepad.

I intend to alter the software for the dew controller at some point to add ASCOM compatibility, not urgent it works fine stand alone but something I'm interested in trying.

Bob

DancesWithWords

24-08-2011, 10:02 AM

Bob,

How goes the build?

frolinmod

25-08-2011, 05:09 PM

I'd like to see something like this where all the smarts are located in software running on a PC connected to the box via USB. The hardware and firmware in the box would only be responsible for sending sense data to the PC, taking control commands from the PC and doing the low level PCM output to the heaters. The software on the PC would have a user interface to change settings and display graphs over time of temperature, relative humidity, dew point, heater output, etc. Oh, I suppose the box could also have a manual mode where everything was controled by the box if/when no PC is connected.

hikerbob

25-08-2011, 08:30 PM

Danceswithwords, started but on hold at the moment.
To much on my plate lately for late nights in the shed (and it's been cold and windy).

Ernie, I suspect that you would still need most of what's in the controller to do that (or an equivalent such as a PIC).

The next step for that would be to add ASCOM capabilities to the controller. I've thought about it often but still not found a good reason other than interest to do so. I wander out occasionally and look at the temp but don't care to much unless I'm really cold or struggling to heat the corrector plate enough to keep dew off.

Bob

DancesWithWords

19-09-2011, 02:39 AM

Bob,

Well I keep and eye out to see if you pick the project back up. In the meantime winter is just around the corner here in Ottawa and I'll be working to more or less replicate your project for use with my XXT12 Orion.

==
Clear Skies
DancesWithWords

Prolixia

31-01-2012, 10:26 PM

Hi Bob,

I was in the process of designing an Arduino-controlled dew heater when I came across your article. Thanks for sharing it - helpful doesn't come close!

I have a question about the temperature sensors. From the photos of the finder scope looks like you slip these under the sleeves containing the heater elements, however I'm worried that if I do that the readings will be artificially high - especially when switching the heaters on for the first time on a cold scope (when the heating element will be hot but won't have had time to warm the scope). Have you noticed this at all in practice?

For an eyepiece/diagonal there isn't much scope to move the sensor away from heating element, so I had wondered about switching the heater off for say 30 seconds just before taking each temperature reading to allow the temperatures to equalise and only adjusting the PWM every couple of minutes (perhaps with a longer initial 'burn time' when the controller detects that the scope is well under the dew point).

Does this sound useful, or am I over complicating things?

Thanks again for sharing your excellent materials!

hikerbob

01-02-2012, 08:55 AM

Prolixia, I've not seen anything to indicate that's an issue. Ideally the temp sensors would be on the optics but I've not thought of a suitable way of doing that. Slipping the sensors under the heaters is the simplest solution I've found but if anyone comes up with a better way of doing that I'd be more than pleased. What I've done is a big step forward from no temp measurements at the heaters but still not ideal.

The only place I've had problems is with a 10" corrector plate, I don't manage to keep the whole thing dry in dewy conditions (but do use the hair dryer a lot less than others). I'd like to add the capability to do a boost on a specific channel, either via ASCOM or using the pot but it's not been a big enough issue to cause me to give it much attention. I suspect the problem is that not enough heat is transfered from the strap to the corrector plate the plate has some tape between it and the housing that supports it so I doubt that tweaking the heater control will fix that. Sometime I'll run the strap without PWM and see if it fixes the dew but that will need a cold winters night.

I'm checking temps every few seconds so turning off the heaters would need a change of approach and reduce the overall duty cycle for the heaters when you wanted them running full bore.

Bob

Prolixia

02-02-2012, 03:18 AM

Thanks for clarifying Bob - that's reassuring.

Regarding your corrector plate, I wonder if that might possibly be a symptom of the problem I was suggesting - i.e. because the heat doesn't transfer quickly enough to the corrector plate the temperature sensor warms up well before it and reduces the output on that channel whilst the collector plate is still cold. That could explain why the collector plate never quite gets up to temperature. I guess you could check by looking at the output to the corrector plate heater whilst it's fogging up - if it isn't high then the actual temperature of the collector plate must be significantly lower than the sensor reading.

I was thinking about possible ways to avoid this issue. As you've said, the best approach would be to put the sensor on/in the part being heated. However, I wonder a sacrifice in terms of the frequency with which the temp/PWM update is performed might permit some level of compensation.

What I had in mind was looking at the current temperature and the desired temperature, and taking a guess at how long it would take to bring the current temperature up to the desired temperature running the heater at full-bore. With a bit of experimentation it should be possible to make a rough estimation for a given component (e.g. the corrector plate). Once you have the results for a duty cycle of 1, it would be simple to scale the results for other duty cycles.

When the controller is first turned on, it's reasonable to assume that the temperature at the sensor will be exactly the same as the actual temperature of the scope part (e.g. collector plate). Using the approximation, the controller now takes a guess at the length of time it will need to run the heater to raise the temperature sufficiently. We could reduce the time a little to avoid overshooting.

Having guessed a the required time, the controller then runs the heater for this time, ignoring any sensor readings. At the end of this, the actual temperature of the scope part should be roughly what we're aiming for. The temperature sensor will probably give a higher reading, since it's right next to the (hot) heating element, so we switch the heating element completely off and let it cool down for a short period (some seconds?). Once the heater has cooled down the temperature sensor should be closer to the actual temperature of the scope part, so we can see how close we got.

We now repeat the process using the new temperature sensor reading. Because we should now be closer to the desired temperature, we possibly don't want to perform the heating quite so aggressively, so we can lower the duty cycle (simply adjusting the time we're running the heater to compensate, e.g. duty cycle halved, time doubled).

The idea is that this heater off -> measure -> guess -> heat process would repeat continuously. The fact that we're only turning the heater off when we expect to have reached the target temperature means that we're not slowing down the heating process as per my original suggestion (you're right - that would slow things down considerably).

This is probably all overkill, but it's a change that could be made entirely in software without needing to change the physical circuit. I'm just thinking aloud really. You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage.

hikerbob

02-02-2012, 07:27 AM

"You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage.You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage."

Might be a workaround for that. In the first iteration I included a current sensor in the circuit. The plan was at startup to run each channel on it's own and measure the current flow and estimate the heater size, eg the bigger the strap the more current they draw.

From what I recall the differences between straps were big enough to know which strap was connected to what.

You could quite possibly get some usable approximations from that for the heating cycles. I found I wasn't using it and dropped it for the second round (extra cost and a little extra complexity). Sparkfun have them. What current measurement would not tell you was the thermal mass of what's being heated or how well heat transfers.

Could well be worth a try, I wanted to keep the unit very generic, all channels behave the same way but that's not an essential.

Might also be worth a look at ASCOM and dew heater control. Not something I've checked any detail on but there has been some work done on that front. Should be possible to tweak individual channels remotely.

Keep thinking out loud, love it as a way of exploring idea's.

Bob

Prolixia

02-02-2012, 08:30 PM

Bob, that's a great idea re. identifying the strip on each channel!

I've only just started to order parts for my controller and strips, but I'm really looking forward to having a play around.

Many thanks,

Prolixia

alistairsam

02-02-2012, 09:05 PM

will it help if the temp sensor was not under the actual heater strip but taped say a half inch away with a slightly longer wire?
in this case, the sensor can be mounted onto the corrector plate than under the heater strip.
for eyepieces, it may not be feasible due to limited space, but I think where there is space available, tape the sensor away from the heater strip.

I've been chatting with Bob on developing a similar controller with the picaxes and will let him post his ideas.
it should be easy enough to have a mathematical relation between the pwm frequency and the target temperature, sort of a temperature regulator to ensure it always stays 5 deg above dewpoint and not more, so you don't waste power.
not sure if Bob's project already does this, will have to go back to the article and check.

hikerbob

03-02-2012, 01:06 PM

The unit's I've built start heating at 5 degree's above due point and ramp up the heating as the sensor's under the heater straps get closer to due point.

One of the idea's that I'd tossed around initially and decided it was looking too difficult with the Arduino was a distributed controller.

Have a master unit to manage the dew point sensing and calculations, display management and comms to a PC if required. Each channel would have a micro controller, mosfet, temp sensor and any other support components required. They could talk to the main controller via 1-wire, I2C or similar. With Arduinos that started to look expensive and bulky.
Using Picaxe 8M2 or similar chips the remote unit's could be very cheap and compact. Eg an 8M2 can be sourced locally for $3.10 http://www.microzed.com.au/index.php?main_page=product_info&cPath=1_2&products_id=219.

The I2C support in the Picaxe's looks pretty good and that should make comms fairly simple and the whole thing could then be very modular. Not sure of the limit's but it should be viable to have a lot of heaters if required.

Waste heat from switching could go into heating (a slight power saving) if the unit's were small enough to fit under heater straps. It could also use an analogue temp sensor - http://www.phanderson.com/picaxe/lm34.html although I don't know what if any benefit's that would give.

I don't yet know the Picaxe's well enough to be certain how to do all that but from what I've read it looks like it should be simple.

Worth a thought for anyone contemplating building a controller as an alternative approach.

Bob

alistairsam

03-02-2012, 02:16 PM

Hi,

I'm working on building a controller with independent pwm channels / distribted controller and as Bob mentioned, the picaxe's should be very cost effective even if individual mcu's are used.
I've used 4 picaxe's on the same I2C bus and they're easy to implement.

I'll carry out some tests and draw up a pcb schematic once I have the prototype working.
I'll try and make it modular so you can add on or remove channels as needed. only bit that will change is the code for the master mcu.

in terms of temp regulation, Bob what would the target temp range need to be at? 3 to 5 deg above dew point till 10 deg above dewpoint or a wider range?
the pwm freq can be linked to the sensor temp so they're not unnecessarily heated but still stay above dew point.

mswhin63

03-02-2012, 03:46 PM

Cheers Hikerbob, :thumbsup:

This was one project I was working on as well, although had to shelve it for the time being. I purchased some Nichrome wire and special silicon high temp flexible rubber strips about 1 year ago in anticipation. Completely forgot about the project after damaging my shoulder.

FYI I am considering using a different PCB design software and ordering package http://fritzing.org/ looks really great for PCB design and small production.

I have both PICAXE and Arduino and may consider one or the other for development. Still mulling over software flow design but this is a good start.

hikerbob

03-02-2012, 06:18 PM

Alistair, I don't have any science to support the choice but I've been working on only heating within 5 degree's of dew point.

Not sure if this makes sense but the relevant section of code is

if ( ChTempVal[TCount] <= dew_point + 5 ) {
ChPWMVal[TCount] = (dew_point + 5 - ChTempVal[TCount]) * 50;
if ( ChPWMVal[TCount] > 255 ) {
ChPWMVal[TCount] = 255;
}
}
else ChPWMVal[TCount] = 0;
analogWrite(ChPWMPin[TCount], ChPWMVal[TCount]); // set the PWM value

ChTempVal is an array storing recorded Temperatures
ChPWMVal is an array storing PWM settings. 255 is fully on, 0 is off.
ChPWMPin is an array storing the output pin number for a channel
TCount indicates the channel

I left some provision to use a switch and the variable resistor (which generally control's the display brightness) to be used to alter the profile but have not implemented that. I didn't allow for altering one channel and not the others. In theory the 5 in the above code would be replaced by a variable and if dew was being a problem with the standard range the variable resistor could be used to increase the temperature that the heating started.

Michael I purchased some nichrome wire as well and had a go at making a heater then went and bought some pre-made thick film heating strips. http://www.dew-not.com/Order_Form.htm

Love them. Home made might be viable in a fixed installation but it began to look risky and difficult for my observatory less equipment.

Bob

hikerbob

03-02-2012, 07:33 PM

BTW, I've ordered some bit's to try a Veroboard version of the remote controller. Not sure how much effort would be involved in getting the existing controller to talk to it but given I'm already using I2C maybe not too bad.

If it works as expected I'd have to look at some options for a surface mount version.

Since I wrote that I've been having another look at the pinout's for the 8M2 and am not sure that I can do both PWM and I2C, the one pin labelled with pwm is also the I2C SDA pin.

Should be able to do a version of PWM in code on another pin. Failing that look into the 14M2 chip's which do have PWM on pins seperate to I2C.

Bob

alistairsam

03-02-2012, 08:02 PM

Hi bob, even if we can achieve independent duty cycles with a chip like the 28x2, we could have four channels off one.

I'll try and test this tonight by measuring frequency with an oscilloscope and a 28x2
You don't have to just use i2c, you could use serin as well but would be a bit more tricky

Bassnut

03-02-2012, 08:06 PM

You don't need multiple picaxes or Pwm. The m2 chips are multitasking (multiple symultaneous programs) and the cycle rate on a dew heater is so low, simple timing is all that's required. With a single non-multitasking 28x2 you could easily control 6 or more heaters.

For one offs, vero board is a hassle. Picaxe proto boards or the kiwi board would be much easier, built in track work for coms and pwr regulator.

alistairsam

03-02-2012, 08:16 PM

I was just reading up on the parallel tasking capability
Since it basically switches between tasks rapidly it might be a bit difficult to get a linear range of 4 independent duty cycles
Other option is just the pwm command that sends bursts
Ideal option is the Hpwm in single mode
Will test

Bassnut

03-02-2012, 08:33 PM

Why bother with hpmw?. Hi freq Pwm just creates noise. A 1 sec or so cycle time is fine and very easy to code with a timer.

The timing variation due to multitasking on an m2 is absolutely trivial.

Bassnut

03-02-2012, 08:50 PM

Just for example, I made a dew heater with a 08m2. The 8pin picaxe, a pot and a FET, just 3 components.
You tweak the pot and the heat changes.

8 lines of code in total. No PWM, no timer, works a treat :P.

hikerbob

03-02-2012, 09:07 PM

Some interesting input. I went PWM in the first place because it was easy with the Arduino and because that's what other dew controllers were doing.

Didn't really think about other options available driving the thing from a microprocessor.

The thermal inertia involved and relatively low power of the heating strips should make fast PWM unnecessary although massively longer heating cycles may alter what the temp sensors see (hot strap rather than the temp being applied to the surface below). As was suggested earlier turning the heater off while temp measurements are taken might be useful there when the measured temp is far enough above dew point.

Bob

alistairsam

03-02-2012, 09:31 PM

Hi Fred

Would you have a circuit diagram for the fet and a part number
I was looking at pwm as that seemed to be the norm but if the frequency is low, just the high and low or pulsout with a variable pause in between is easy enough.
My objective is to keep heating to a minimum to conserve power when running on a car battery, hence bob's idea of calculating dew point and regulating heat.

Bassnut

03-02-2012, 10:10 PM

I lied, 4 components :P.

The FET is a MTP3055V, good for 12 amps !!. But any digital N channel FET would do.

Dew point calculation is very clever to save power, would also need an ambient temp sensor. Could be easily handled by even the 08m2 and a bit of code.

Bobs article is a work of art though, a whole different ball game and very comprehensive.

hikerbob

04-02-2012, 09:13 AM

Fred thanks for your kind words. I'm somewhat proud of my heater (although I look forward to it being improved on or superseded).

"My objective is to keep heating to a minimum to conserve power when running on a car battery", when I'm not on main power I often run off one of the smaller SLA batteries used in burglar alarms etc. Minimising power usage goes up in importance.

I've been using a cheaper temp/humidity sensor in more recent play, I think I mentioned it elsewhere but just in case http://www.techtoys.com.hk/Components/SHT1x/SHT1x.htm

Somewhat cheaper than the SHT15 (http://www.sparkfun.com/products/8257), a bit less accuracy but for what we are doing I can't see that matters. There are other alternatives around for the humidity sensing but I've not seen anything quite as neat for the purpose as the SHT sensors.

Bob

mswhin63

04-02-2012, 12:00 PM

Hi Bob,

Using a Temp Humidity Sensor is a better play as I was considering this for fully automatic dew heater. I feel there is no requirement for ASCOM compatibility as it should find the sweet spot for dehumidification, Unless someone can tell me otherwise, maybe something I am missing.

hikerbob

04-02-2012, 12:59 PM

Malcolm the only credible use I can think of for ASCOM (or similar) is the ability to tweak individual channels without making the panel on the controller more complex. Overall I like having the controller sitting there doing it's thing without the need for me to bother with it once it's turned on.

If you got really keen maybe the ability to use the temp sensor already on the scope to tweak focus as temperature changed but from the little I saw of that I suspect that it might create more problems than it solved.

I've also thought about logging humidity and temperature during a session but suspect that I wouldn't bother looking at the results so have not done anything about that.

Bob

mswhin63

04-02-2012, 01:14 PM

Very sound logic, I think we can overload ASCOM is bit too much sometime. I have always though some ASCOM drivers like Dome could be done with so much more simplicity instead by using discrete component or at most Arduino.

alistairsam

08-02-2012, 04:16 PM

Hi,

I went to jaycar and they mentioned the MTP3055V is discontinued.
I got the P16NF06 instead. datasheet here.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002501.pdf
the test circuit for resistive loads is attached, but should it be wired as per Fred's diagram or as per the test circuit?
I don't get what Vd is. Would the mcu output be connected to input of Rg?

As for the issue of temp sensors being under dew heater straps, what if we used a piece of stryofoam as a thermal insulator between the strap and the sensor so the sensor contacts only with the metal of the scope?
you'd need temp sensing to keep heating regulated and save power.

I also got some nichrome wire. Its this one http://jaycar.com.au/productView.asp?ID=WW4040&keywords=nichrome&form=KEYWORD

I wanted to make a strap for a 2" eyepiece barrel for my Qhy8 and guidescope.

for a power of 2W, the current would be 180ma.
for 180ma, Resistance required would be 66ohms.
is this correct?
if so, the wire I got is 13ohm per metre, would I need to use 4metres to get close to 60ohms?
I might just get a dew strap but thought I'd try the nichrome as well.

should get a unit based on Bob's concept built soon. I love the idea of an automated and efficient controller that conserves power intelligently based on real time feedback
and going by Fred's comments, if pwm is not required we could easily build a 1 chip 28x2/20M2 controller with 4 or more channels just using pulsout as you can define the width of the pulse in the command and each channel can be independent based on its sensor.