Tom I've used Dallas 1 wire sensors for the strap temp measurements I've had a sensor wrapped in a small heater and I've not seen anything to suggest that the RFI is intefering with it.
I used the Sparkfun breakout board for SHT15 for the ambient temp/humidity sensor http://www.sparkfun.com/commerce/pro...oducts_id=8257
Voltage sensing is a resistor divider circuit feeding an AD port on the Ardunio and the current sensing is a Sparkfun breakout for ACS712 http://www.sparkfun.com/commerce/pro...oducts_id=8882 (I've not got that working correctly yet).
I'm using A Sparkfun Serial LCD unit http://www.sparkfun.com/commerce/pro...roducts_id=812
Some overkill in this but I'm using it as a learning exercise, I've got a couple of other temp control projects in mind.
I'll attach my code (as is), as mentioned the current measurement stuff is not working properly and since adding in the current and voltage sensing I seem to have messed up the LCD (I suspect a connection problem).
I have already built all of my dew heater straps with an integral LM335 - before I new of the Arduino and the 1 wire temperature sensors. If I stay with the LM335, to keep ports free on the Arduino, I am thinking of multiplexing the inputs. Will likely use 3 straps at a time - corrector plate, eyepiece, x1 finder (Telrad or Rigel). My 4th output will be used to power a heater in my eyepiece case (will have to build an insulated eyepiece case). I have aquired a Humirel RH sensor but was looking at the Sparkfun SHT15 as an alternative as the interfacing is already in place.
Have you given any thought to the PWM code? Such as what conditions to change the PWM timing versus just on and off? I was contemplating trying to have the Arduino set the PWM timing so that each individual heater was on all of the time but just enough to reach the dew point plus a degree or two. this would allow for a more predicatable overall battery drain. the other scenario I was looking at was to intersperse the PWM over the heaters such that only one heater was "on" at any one time. I was trying to figure out which would be most efficient but have not as of yet ran the numbers.
Once I get the current sensing working properly I hope to use the current flows for each output to estimate the size of strap attached (and hence the size of the optics) and alter the behaviour of the channel to suit. I'm expecting that I might want to start heating a bit sooner and ramp up more quickly.
During the initialisation stage each channel is turned on individually and briefly and the current measured and recorded.
I've not put much thought into managing the peak power load yet, so far it's been a back of the mind thing. Once I get the electronics right the rest should be a matter of tweaking the code based on experimentation.
I've done some more work on the code for the Ardunio based dew heater controller and now have the current and voltage readings right for my setup. I want to add a brightness control for the LCD backlight and still want to do some work on the behaviour of the heater control for different sized heaters. I may also do some more work on what displays when in the LCD.
I gave the heater controller a good run last night. I hooked 4 heater straps up 12", 2of2" and .96"and ran the whole thing of a selaed lead acid battery (the sort that goes in burgler alarms etc).
For most of the evening the ambient temp was a couple of degree's above the dew point but with the 5 degree margin I use so the heaters were running at a light duty cycle. With a dew point of 17 and ambient of 19 the sensors just under the heater straps were reporting a temp of 22.
The battery did not seem to have any trouble keeping it all going and showed only marginal voltage drop under load. The optics all remained dew free while others were having dew problems.
The ugly part was the wires running all over the scope, I do need to find a way to tidy that up.
Have been giving the wire routing some thought over the last few monthes. I have decided to make my strips with a short leads and permanently install jacks and wiring on my scopes.
Have been looking at using RJ style connectors (the pins are surprisingly rated at 1A!) mounted in close proximity to the point of use. This will allow one bundled or multiconductor cable to plug into the Dew Controller which I am mounting in or on my power center.
For my Newt, the primary and secondary dew heaters are permanently mounted already. The eyepeice and finder heaters are already in close proximity to each other so one small 2x jack panel will do the trick. I will likely add a bracket to the base to accomodate mounting the power center (on the front of the base).
For my Mak/Cass, the power center will sit under the tripod and again the wiring will be instaled permanently onto the scope with velcro attaching it to a tripod leg. This time there will be a 3x jack panel to for the eyepiece, finder and corrector.
I had decided on RJ-45s as they inexpensive are are widely available (CAT-5/6 cables), and have 8 pins (2 for +, 2 for -, with 4 left for sensor data and power).
I have just taken my first steps into the 'dark art' of electronics, with the goal being to construct the project that is described in the original post. I have put together a couple of the Jaycar Short Circuits Series kits. The soldering was pretty rough to begin with but it improved fairly quickly.
I now feel ready to have a go at Motor Speed Controller (Jaycar kit KC-5225). I still have to work out the details for the resistor dew straps but I have a few areas to learn before I will understand what is needed prior to tackling that part of the build.
I think Jaycar is now my favourite store. So much great gear. I have to say that some of the customers looked like they walked off the set of Big Bang Theory.
My thanks to Al for making the information available.
Last edited by Shark Bait; 04-11-2012 at 05:47 PM.
Thanks Al. I used your sheet as a starting point and applied your calcs to Nichrome. Worked out well. PWM is from an Arduino with a humidity sensor proving the input. Basically, PWM is scaled to humidity input.
I have taken the plunge and constructed the 10A 12VDC Motor Speed Controller (Jaycar Kit KC-5225).
I thought I was ready to give this project a go, being the first time that I have moved away from following a kits instructions to the letter. Some of the kits components have been updated and that threw me. Initially I felt a little out of my depth, but after reading the papers a few more times things fell into place.
The first photo shows the testing using a 12v red LED with pot turned to maximum. The second is with the pot turned to minimum. It appears to work. I had some trouble soldering the power leads to the terminal posts. That part will have to be tidied up before it all goes into a jiffy box.
Jaycar had sold out of Nichrome Wire, so the dew strap for the primary will have to wait. In the meantime, the smaller dew straps to be constructed from resistors can be started.
If I can do this, anybody can. It's a very satisfying way to spend the evening after the kids have gone to bed and the clouds have ruined the nights observing.
Last edited by Shark Bait; 25-11-2012 at 07:57 AM.
The heater board, RCA sockets, 12v red LED and associated wires have made it into a box to reduce the chaos. The dew heater straps will have to wait for another day.
I am using a 12VDC power supply, so I have used a link in place of the 12 Volt regulator as supplied with the kit. This is in accordance with Brendan's addendum to Al's original design. Brendan (wasyoungonce) discussed that this design needs 'headroom' voltage.
I have measured the voltage at the RCA sockets as it appears in the photos. With the dial turned all the way clockwise, the LED is dim and it reads as 1.95 Volts. A maximum of 12.5 Volts is reached by the time the pot is only half way through its travel and that is what it reads until the pot is turned all the way anti-clockwise.
This is the point where my lack of knowledge in this area lets me down. Any advice would be gratefully accepted.
Is the 1.95 Volts the 'headroom' voltage that I should expect to see?
When I construct the 4 dew straps and there is a larger load, can I expect to see 0 Volts as a minimum and the maximum voltage achieved when the pot is turned all the way anti-clockwise?
Thanks.
Last edited by Shark Bait; 25-11-2012 at 04:07 PM.
.....
I am using a 12VDC power supply, so I have used a link in place of the 12 Volt regulator as supplied with the kit. This is in accordance with Brendan's addendum to Al's original design. Brendan (wasyoungonce) discussed that this design needs 'headroom' voltage.
................ With the dial turned all the way clockwise, the LED is dim and it reads as 1.95 Volts. A maximum of 12.5 Volts is reached by the time the pot is only half way through its travel and that is what it reads until the pot is turned all the way anti-clockwise.
This is the point where my lack of knowledge in this area lets me down. Any advice would be gratefully accepted.
Is the 1.95 Volts the 'headroom' voltage that I should expect to see?
When I construct the 4 dew straps and there is a larger load, can I expect to see 0 Volts as a minimum and the maximum voltage achieved when the pot is turned all the way anti-clockwise?
Thanks.
Stu..the Headroom voltage is head room for the +12V regulator only. What this means is that for the 12V regulator to actually regulate voltage then the input voltage to it must be around 2.5V (actually some times 2.2V) above the voltage it is rated to regulate out.
For example a +12V regulator needs around 14.2 ~14.5V DC in (minimum) for it to regulate the output efficiently to +12V DC.
If the input to the same +12V regulator was lets say 12.5VDC ...then the regulator cannot regulate the output voltage efficiently, it may work but it may not and it is outside the mfgr's operating specifications.
Thus these type of 3 terminal regulators need an input voltage headroom above what their rated output is. It the case of the PWM controller you built...it was designed to operate a number of motors (or whatever) from a varying range of input voltages. But since most of the Astro guy's (and Gal's) are using +12V DC supplies already ...then you don't need the regulator, just install the link shown in the addendum. I see you have already done that so you are all thumbs up! (note: if you using a higher DC supply like 18-24V then you will need the regulator).
The voltages you are measuring at the RCA are the minimum and maximum range of PWM (avg output) the controller can attain (due to it's design)...nothing wrong here.
Last edited by wasyoungonce; 25-11-2012 at 07:17 PM.
Is the 1.95 Volts the 'headroom' voltage that I should expect to see?
When I construct the 4 dew straps and there is a larger load, can I expect to see 0 Volts as a minimum and the maximum voltage achieved when the pot is turned all the way anti-clockwise?
Thanks.
The 'headroom' voltage referred to probably refers to the fact that when you are using regulators, you are required to supply it with a couple of volts over the regulated voltage. Even when using low dropout regulators, a volt or so extra is usually a good idea - the data sheet will tell you.
The measured voltage at a PWM output depends on the particular circuit. Some PWM outputs will range over a (say) 5% - 95% duty cycle - not necessarily 0% - 100%. If this is the case, then you will get a small voltage at the 'minimum' setting, and near maximum voltage at the maximum setting. In your case, with the LED at max when you have the setting near 50%, it may depend on the series resistor you are using with the LED. The average of the PWM voltage at your 50% setting may be providing sufficient current to fully illuminate the LED.
By the way - you would normally wire the PWM control so that the max output occurs when the pot is fully clockwise.
A neat way to do posts is to wind 3 or 4 turns around the post first.
Good idea. That should tidy things up a little. Still, it will be a while before I will try soldering anything really delicate.
Quote:
Originally Posted by wasyoungonce
Stu..the Headroom voltage is head room for the +12V regulator only. What this means is that for the 12V regulator to actually regulate voltage then the input voltage to it must be around 2.5V (actually some times 2.2V) above the voltage it is rated to regulate out.
The voltages you are measuring at the RCA are the minimum and maximum range of PWM (avg output) the controller can attain (due to it's design)...nothing wrong here.
Thanks for the detailed reply Brendan. I understand what is going on now. This little project has been an eye opener.
Quote:
Originally Posted by Scorpius51
The 'headroom' voltage referred to probably refers to the fact that when you are using regulators, you are required to supply it with a couple of volts over the regulated voltage. Even when using low dropout regulators, a volt or so extra is usually a good idea - the data sheet will tell you.
The measured voltage at a PWM output depends on the particular circuit. Some PWM outputs will range over a (say) 5% - 95% duty cycle - not necessarily 0% - 100%. If this is the case, then you will get a small voltage at the 'minimum' setting, and near maximum voltage at the maximum setting. In your case, with the LED at max when you have the setting near 50%, it may depend on the series resistor you are using with the LED. The average of the PWM voltage at your 50% setting may be providing sufficient current to fully illuminate the LED.
By the way - you would normally wire the PWM control so that the max output occurs when the pot is fully clockwise.
Good luck.
Cheers
John
Cheers John. I still appreciate the time taken to educate a beginner. I do have a better understanding about what is going on now.
I can only blame Mr Squiggle for the PWM control being back to front / upside down. If you ever need any electonic soldering done, send it my way. I can safely guarantee an end product that will be unique ......... I can't leave it that way. I will have to fix it. It is going to annoy me every time I use it if I don't.
