I needed a dew heater, so thought it was a good time to revisit the excellent Dew Heater controller built in IceInSpace 2010 by Bob Stephens. I had two reasons for doing this:
1. There are some great modular arduino products which make it extremely easy to make. The Nano has a great extension I/O board so all the components CAN BE attached with pre-made connectors. There is NO circuit board making. There is NO soldering onto circuit boards. The ONLY soldering is for the plugs and sockets to connect it to the outside world.
2. Arduino based modules are now very cheap. Apart from the heater straps it costs ~$40 for a 1x channel build (Jan-Feb 2016).
3. Except for the heater it uses very little power. In fact, you can leave it on all the time to monitor temp/humidity – only plug in the heater if needed.
The 1st attached pic show it connected and its inside connections
How the controller works & software The controller is similar to the build by Bob Stephens (with additional liberation of software from other sources) so I won’t go into details. It is Arduino based and feedback controlled, briefly:
1. DHT22 senses ambient temperature & humidity. The dew point is calculated.
2. DS18B20 temperature sensor placed inside the heater strap senses the temperature of the element to be heated. You’ll need 1x for each bit of glass to be heated. I haven’t done the code for multiple channels yet, but will do so if any interest.
3. Calculate the difference = ( heated element - ambient dew point ).
4. Send a PWM output from the Arduino based on this difference. I have used a threshold for activation of the heater of 5oC above ambient dew point (this is what most seem to use - can be changed in the software). So the PWM output = 0% at +5 oC (and above this) to 100% at 0 oC.
5. This PWM output drives the heater strap via a MOSFET driver module. The heater output increases as the temperature difference between heated element and ambient decreases.
6. The display swaps between showing (1) the ambient temperature, humidity and dew point and (2) the heater temperature, difference to dew point, and heater drive level.
The parts The parts as follows. The 2nd attached pic shows the main Arduino bits.
• Arduino bits ($10-11 – ebay O/S vendors): Arduino Nano; Nano V3.0 prototype shield I/O extension board expansion module.
• The sensors ($10.50 – ebay O/S vendors): DHT22 Digital temperature/humidity sensor; DS18B20 digital temperature 1-wire sensor (need an extra DS18B20 for each additional heater). The beauty of these is that they are digital – they are plug and play as they do not need calibrating.
• Display ($7 - $12 – ebay local & O/S vendors). I used a 0.96” I2C SPI serial 128x64 OLED display - it is small, so if you only want 1x heater channel it all fits in a small light (box) that I Velcro onto the scope. You can use other display eg 20x4 red LCD display. Any display is fine just as long it is an I2C serial display as it only require 2x connection leads (it wont use up all of the Arduino analog/digital ports if you need multiple heater channels) plus 2x power leads.
• MOSFET driver for heater (ebay O/S vendors). $3.50 1x channel module and ($10 for a 4x channel module).
• Hardware (Jaycar). ~$10-15 for Jiffy box, plugs/sockets, wires and 1x resistor.
• Heater strap (Jaycar & Bunnings). Whatever you want. I won’t describe the heater straps here – I use nichrome as its much easier than resistors (and not difficult to connect to wire as some would have you believe).
The nice thing about the extension I/O board is that each Arduino analog/digital/I2C input/output has pins for that input/output & +5VDCC/GND. So you can just plug the pre-made connectors into the IO board. For the MOSFET driver and display just plug the other end into that module. For the sensors (DHT22, DS18B20), just cut one end off and solder onto the appropriate lead or socket (and cover with heatshrink = wonderful stuff).
The circuit The circuit and how you put it together as follows (shown in the 3rd attached pic). A few things to note:
• The DHT22 attaches to the outside of the box. So get 3x connectors, cut off one end, solder to the DHT22 leads and cover soldering with heatshrink (1.5 – 2.55mm). See the 4th attached pic.
• Have used parasitic mode for the BS18B20 (so only need 2x wires instead of 3x). So you can’t go wrong wiring it up. Just connect the 2 outside ones together – these go to GND; the central one goes to the signal of digital 12. If you cover up all the solder joints and its connections to the 2 core wire with heatshrink it is nice and watertight (see 4th attached pic). There might also have to be a one line change to code in the library of you find it doesn’t work (I can provide details).
• The serial display – just needs 4x wires. Don’t forget to swap over the SCL & SDA connections between display and extension I/O board.
• MOSFET driver connections = easy.
• Power – run the 12V input to both the Arduino extension IO board and the MOSFET driver.
Getting it going Before you do the whole build its best to put it together in stages and test each bit.
1. First put Arduino on your PC or Mac, load up the attached sketch (it will need a few libraries uploaded – should happen automatically). Don’t worry if you haven’t used Arduino before – its not too hard.
2. At this stage its best to put it together in stages to check it works. (1) plug in the DHT22 and check that it reads the ambient temp/humidity. (2) plug in the Ds18B20 and check that it reads heater sensor temp and calculates the heater output. (3) Plug in MOSFET driver and heater strap – does it heat it.
3. Put it all together in a jiffy box.
Your definition of "easy" may not be agreed upon by everyone... The instructions that you posted look complicated enough. Nevertheless, it's going in my TODO bookmarks folder. Thanks!
I will bookmark this page, it looks a great little project, after I manage to do a stepper controller for a moonlite focuser. May take a while, so hopefully the page doesn't disappear.
Thanks for putting this up, will save the zip file for future references..
Just depends upon the current they draw. I've used IRF520 MOSFET module. This can do 1amp as is, and up to 5amps with a heatsink. I've used it on a 4" refractor and an 8" SCT and its fine without a heatsink - but conditions don't require a massive heat drive in Sydney. The rule of thumb is that if the mosfet feels too hot then it isn't coping - so add a heatsink or go for a higher power module.
I couldn't find the specs on the Kendrik straps. But the Astrozap 4" heater is 0.6A, 6" is 0.8A and 8" is 1.2A - so it will be fine with those. Just look up whatever heater strap you have and see what current it draws then compare to the above IRF520 specs. And don't forget - most of the time the heater will be running on a low duty cycle if conditions aren't terrible. So if the heater is running 50% duty cycle this equates 0.6A for the above 8" strap.
The thing about the setup I have described is that it uses simple interchangeable modules, so if you need higher power then just get a higher power mosfet module. I'll have a look for some higher power modules when I get a chance. But really, if your need to draw more than 5amps - thats 60W ! Might as well pull out a hair dryer !!!!
Thanks for your interest. Chris
Quote:
Originally Posted by DavidTrap
Hi Chris,
Can dew not or kendrick dew straps be used with this device??
I'd be looking at this for a warming the mirror of a 10inch RC. The strap wraps around the inside edge of the tube and heats the mirror by radiant heat.
I only want to keep the mirror a degree or so above ambient dew point.
The current draw would be 1.6 amps at maximum. Any suggestion of what model Mofset I would require? (Much to learn about all this stuff!)
The funny thing is, I have made one almost exactly the same as yours. As I was reading your article I was thinking, that sounds familiar.
Same temp sensors and humidity sensor.
The difference with mine is I am using the Freetronics N-MOSFET Driver which can handle up to 40 Amps so pulling up to 3 amps, does not even get hot. Also have 2 channels.
I have also taken advantage of the USB connection and do stats that can be imported into Excel to show all parameter info. That way the unit can be monitored all night to check the performance. As I'm always using the laptop, I have written some software to show all the parameters live.
Apart from fully auto mode, I aslo have a manual mode built in where 4 different settings for each channel can be set. Also these settings can be custonised to whatever suits the users scope/location. To top it off, it starts to the sound of Black Dog by Led Zeppelin!
I use 2 LED's to display the modes and that also helps to keep things light as it's mounted on the tube.
Great minds think alike. I'll post details of mine when I get a chance.
I like the Zeppelin touch ! What about Sabbath, Pigs of War, with them coming out for a tour ?
Manual mode - good idea.
I was after an easy build that wouldn't scare people too much.
Love to see your build. Yes, it takes as long to write it up as design it.
Quote:
Originally Posted by redbeard
Hi Chris,
Well done with this device.
The funny thing is, I have made one almost exactly the same as yours. As I was reading your article I was thinking, that sounds familiar.
Same temp sensors and humidity sensor.
The difference with mine is I am using the Freetronics N-MOSFET Driver which can handle up to 40 Amps so pulling up to 3 amps, does not even get hot. Also have 2 channels.
I have also taken advantage of the USB connection and do stats that can be imported into Excel to show all parameter info. That way the unit can be monitored all night to check the performance. As I'm always using the laptop, I have written some software to show all the parameters live.
Apart from fully auto mode, I aslo have a manual mode built in where 4 different settings for each channel can be set. Also these settings can be custonised to whatever suits the users scope/location. To top it off, it starts to the sound of Black Dog by Led Zeppelin!
I use 2 LED's to display the modes and that also helps to keep things light as it's mounted on the tube.
Great minds think alike. I'll post details of mine when I get a chance.
I'll have a look for some higher power modules when I get a chance.
Thanks for your interest. Chris
Hi Chris,
Just wondering if you've had a chance to find any higher power modules. I'm a complete novice at this sort of electronics, so really appreciate any assistance you can offer.
Can you suggest a heatsink for the MOSFET module pictured in your build? I'd like to be able to cope with a 2 amp load through the dew strip.
I had a look at the N-MOSFET that Damien suggested. I'm not sure if that is compatible with your project - I think it triggers differently?
The IRF520 module will be fine at 2A if heatsinked.
Get a TO-220 Heatsink (6021 Type) from jaycar. Its $1.45, plus paste and washer.
I'll measure what mine is doing over the next few nights - with heaters for 4" and 8" scopes.
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.
Quote:
Originally Posted by DavidTrap
Hi Chris,
Just wondering if you've had a chance to find any higher power modules. I'm a complete novice at this sort of electronics, so really appreciate any assistance you can offer.
Can you suggest a heatsink for the MOSFET module pictured in your build? I'd like to be able to cope with a 2 amp load through the dew strip.
I had a look at the N-MOSFET that Damien suggested. I'm not sure if that is compatible with your project - I think it triggers differently?
Chris thanks for the kind words about my older design. Glad to see that it continues to be of use. Thanks also for sharing what you have done and the benefits that brings to others. Love the power of the web to enable us to share and build on others work (as much of mine was done by pulling other stuff together). Bob
HI again to all,
Sorry about the late reply.
I've added some pics to show what I've done.
How it works is fairly simple:
With the switch in the manual position, the unit will use presets that have been pre designated. In one of the photos, you can see the heater settings can be set for each channel and each channel has 4 heater settings that can be selected by using the small push button. These settings get downloaded and saved in the eeprom.
By pressing this button in, it will cycle through the different heat settings and beep and flash to let you know which setting has been selected. 1 flash and beep for level 1, keep holding the button in,,,2 flash and beeps for Level 2 .......etc. When using the software, it will display the selected settings. The unit does not need the software to run, it's stand alone, but the software is a bonus.
When the switch is in automatic mode, the unit just works and uses the sensors to detect the ambient temp, RH, and heater temperatures to set the heater output PWM. My favorite setting.
The software is fairly straight forward in respect to features, and multiple files can be saved with different settings, (perhaps for different locations).
The unit is very light itself which is great for mounting on the telescope tube.
You can use any software to read in the values as the data is simple and easily imported.
The Humidity/temp sensor on the top, I just wrapped white electrical tape around it for a little protection.
There are a few prototype holes drilled where they should not be, lol.
The centre connection on the front has 2 RCA connectors for the dew heaters and the central connector is for power. I love these power connectors and try to fit to all my gear as I have several 5 amp power supplies with the same mating connectors. The 2 LED's below this are the indicators.
The rear connections are for the 2 temp sensors with the USB in the middle. Below the USB is the small button for selection and the switch below that to select modes. Always use a fuse somewhere logical in the power feed. I have an inline one I use, not shown.
I use DEW NOT heaters as they are reasonably priced and work well.
The photos show the software, (excuse my galaxy pic, couldn't help myself), in operation as well as the settings form.
I've really appreciated reading about your project and I'm guessing you got a buzz when it was working as I did. This is fun stuff.
There is also another guy here on IIS, (Garbz), who has also built an amazing all things device. Look it up. He really knows what he is doing and helped me with some advice on saving memory!
