DIY smart dew heaters controller

[Vroobel]

Hi all, 

Recently I bought a quite nice Vixen A80M, and now equipped with driven EQ5, quality diagonal, OVL bino and some pairs of EPs I would say I'm ready to watch the Moon, planets, doubles, etc. But I cannot... 

I'm not sure a rest of the UK, but Liverpool is very moody in terms of weather. Since the New Year I had literally 6-7 good nights. But even if the weather seems good, the dew kills the fun.

So I decided to build a 4-channel Arduino based smart dew heaters controller with DHT22 and NTC thermistors. Additionally I'm going to join a feature of the dew heaters controller with a moto-focuser controller, which is already working (as prototype). I use the Arduino Nano, so I have limited number of IOs. I have a plan to use a higher capacity rechargeable battery, because I want to make the refractor totally portable. A case for the multi-function controller will be 3D printed. 

My idea is to keep constant difference between a temperature of the heaters and an ambient temperature and I can easily do it using a PID_v2 library and some NTC theory. That works very well and I can control the difference of temperatures by potentiometer in range 0-20*C. 

I have two questions at this moment:

1. Is it really reasonable to base the algorithm on a dew point instead of my current solution? Or maybe I can switch the controller between the two options if/when needed? I know, how to calculate the dew point, but how the controller should work based on that? 

2. How big the difference between temperatures of heaters and ambient should be? As I wrote, I can set it between 0 and 20*C now, but I can expect that too small difference is not sufficient while too big difference causes heating air over lenses affecting seeing. What is your experience in this matter? 

I found that some people don't need to use heaters with EPs, but in Liverpool I experienced the dew on EPs many times. 

BTW, today I found this project:

Accidentally we had similar ideas, but my controller won't be so high-end. Despite this, if someone is interested in copying my simpler ideas I'll share them. 

Thanks for any support 

Tomasz

Edited July 25, 2022 by Vroobel
Grammar, syntax.

[kbrown]

Watching with great interest. I haven't progressed much with my version. Too busy with other things.

Originally I thought of NTC thermistors too but with them you need to take into account wire lengths or go with 3 or 4 wire configuration. That's why I decided to use digital sensors instead. Drawback is more coding involved and the poor Nano has very limited ram.

[Vroobel]

Hi

The controller is finished now and I can share some facts and pictures.

As you can see, that's a form of hat for a small 12V 30Ah rechargeable battery, like half the size of one for a petrol car. All design is made in a FreeCAD, I like this program, it's free and not too hard. This construction allows easy disconnection for charging. Once fully charged battery allows at least 3 nights of fun. It should be very cold and wet at night and I should use 4 heaters fully driven to significantly limit the time. The goal of reasonable mobility (using car) is obtained.

Power switch, 4x20 chars LCD, encoder with micro switch and digital voltmeter including buzzer informing about drop of the voltage below individually set level. The 12V voltage provides excess over the need, as the Arduino drives 4 MOSFET transistors using pulse width modulation (PWM), so 9V or even 7V should be enough to drive the heaters. I don't know yet the efficiency of the set, when the voltage drops so much.

The encoder is a great thing, rotation with explicit jumps allows to precisely increase or decrease the setting of deltaT over the dew point. The minimum reasonable setting is +6°C, but I let it change up to +20°C. Pressing the axis of the encoder I change an active channel, so I can set a different temperature for each heater. One picture below shows the selected channel by asterisk (*), while the value of deltaT is also displayed for the exact heater. The picture shows 3 heaters at work with settings 1:+10°C, 2:+15°C, 3:+20°C. The fourth channel is not occupied now. Why did I decide to be able to set different temperatures? Recently I checked it in practice, the lens of my Vixen A80M was fine with +6°C over the dew point, while BST eyepieces in the binoviewer were getting foggy because of extra humidity from my eyes!  I had to increase the deltaT significantly (I don't remember, 15-20°C) to remove the dew, but in the meantime I heated up the air over the lens. 

Two USB sockets provide 5.1V power for the purpose of the RA drive in my EQ5 mount. Two aviation GX sockets are there for remote control and stepper motor of home made moto-focuser (another story, but the same Arduino and the same battery work for it). I like the GX connectors for their variety and stability. There is also an Adafruit BME280 weather sensor in front (instead the DHT22 mentioned in previous post, I changed mind) and a USB extension for the Arduino, so I don't need to open the box to improve my code.

The code supports possibly every variant and order of connection and disconnection of the heaters and NTC sensors. This is against accidental tearing the cables out or if I forget to connect something and decide to do it later. The sensors are placed inside of heating bands. It's good to know that the NTC thermistors, which I used, has reversed characteristics, so if the circuit is open, the PID algorithm takes it as very high resistance that means low temperature and it strives to heat the heater up (https://www.sensortips.com/featured/what-is-the-difference-between-an-ntc-and-a-ptc-thermistor/#:~:text=Both negative temperature coefficient (NTC,allows current to pass through). The PID_v2 library is quite universal, but I didn't have more time to dig in it.

I'd like to mention that the case is designed in a way to run the dew down, so it's safe.

What can it do?
- readout of an ambient temperature, relative humidity (and pressure of the air, if needed),
- calculation of the dew point,
- driving independently 4 channels using PID and PWM,
- allowing of independent setting of temperature for each channel between 6 and 20 °C,
- measurement of the battery voltage with informing (buzzer) about dangerous drop of the voltage,
- providing 2 x USB power,
- steering of moto-focuser based on LV8729 driver (or any in Pololu standard) and Nema 14 stepper with 5:1 planetary gearbox.

The gadget was recently tested in its initial version during my first hunt for the planets, but I added some features, so I cannot wait for a clear sky now. 

BTW, regarding the additional drop of voltage on wires of the NTC, I took it into account and I think that the total resistance of the NTC and its wires is measured in the readout. I think I shouldn't care too much as the PID watches the process and heaters do the job. Let's check ASAP!  

 

Edited August 3, 2022 by Vroobel
Additionanal text.

[kbrown]

Very nice! Well done! Is that battery a deep discharge lead acid battery? If it's a normal car battery then I'd keep it topped up frequently to prevent sulfation.

[Vroobel]

Oh, I'm sorry for so late reply.

The Halfords writes: The Yuasa U1R Specialist and Garden Battery is designed for ride on mowers, kit cars, garden tractors and specialist applications. Its rugged construction resists vibration and deep discharge damage,the L-type terminal allows quick and easy installation. This battery is supplied charged and ready to fit, it is fully maintenance free with advanced safety features.

So I hope, I can easily work till the voltage doesn't drop below 12V. The voltmeter, which I used, has buzzer set on the 12V threshold. But my experience says that once fully charged I can play 2-3 nights, depending on the heating need. The stepper motor used for the moto focuser get's the Enable state switched off 1sec after its use, so the power consumption is really minimal.

Edited August 27, 2022 by Vroobel
Grammar, syntax.

[Vroobel]

Hi, 

It's pointless to take the battery-powered smart dew heaters controller every time I want to capture DSOs using my 90kg EQ fork mount. Therefore I decided to build a smaller version of the heaters controller without the focuser control feature. It will be powered from 12V like other elements of my setup. The same built, the same code for the Arduino nano. 

 

[Vroobel]

Here is how the smaller version of the dew heater controller looked during assembling and straight after.

 

Edited February 13, 2023 by Vroobel

[EarthLife]

Very nice !

Just as a matter of fact (I expect you already know anyway), you can now get 10 (I think 4-layer) PCB's, silk screened etc made (up to 10cm square) for like £15 inc vat/postage total - in case anyone prefers to have a nice looking non-prototype result.

[Vroobel]

Yes, I know they are cheap now, I'll consider it in future. I don't do it for sale, so at this moment it's not important how it looks on the board.

Years ago, before that technology appeared in the marked, I made plenty of custom circuit board on my own. Initially I used an oil marker to create the paths to replace it with set of liquids: one to spray a laser printout to make the white space UV-transparent and second to cover a raw copper laminate. After exposing the laminate through the printout on UV rays the uncover area was easy to etching in nitric acid or later in ferric chloride. Now we have so affordable technology, but I'm simply happy to make my circuit working.

Edited February 13, 2023 by Vroobel