Arduino dew heater control

[kookoo_gr]

Also forgot to mention that the ST pin is zero so there is some sort of failure

[dtokez]

I'm back in business, the contrast pot was faulty after all! Duty cycle is working correctly now, and the outputs can be driven with total control. So, its back to the dome later to get it all hooked up again.

I hope to be able to report tomorrow that all is well and good.

Phil

Nice build! Just a quick note, I never use pots for contrast adjustment, well I used one to work out what value worked best then I just add fixed value 0805 resistors to the LCDs themselves to save on some wiring

[Gina]

I'm looking again at dew point controlled dew heaters so coming back to this thread

[brown_rb]

There is one at 

http://sourceforge.net/projects/arduinonanodewcontrollerpro/

with schematics, code, data logging, remote function, unmanned operation etc

[AstroShomy]

Gina, do you think this project could be simplified by eliminating the dew sensor? We actually don't need a dew point calculation. I think it would be enough to measure the ambient temperature and heat the optics a few degrees above it, regardless of the dew point.

[Gina]

Gina, do you think this project could be simplified by eliminating the dew sensor? We actually don't need a dew point calculation. I think it would be enough to measure the ambient temperature and heat the optics a few degrees above it, regardless of the dew point.

Interesting thought   I don't know for sure but I think the humidity variation would be the main variable controlling the power.  With fixed heating the optics will be a few degrees above ambient.  This is what I have been using but there are times when it's dry and dew heaters are not needed and others when there's a heavy dew and more heat is needed to clear it.  You can have manual control and just judge it but the automated system isn't that much more complicated for a DIY project.  In my case the Arduino Nano is already there for remote focussing.

[Gina]

This is the single scope version I'm considering for my MN190.

[Gina]

A few notes about the above circuit diagram as applies to the dew heater control...

I considered various devices for the dew heater control.  I chose power MOSFETs to switch the heater current on and off - in fact using PWM from the Arduino for variable power.  These are cheap and have a very low ON resistance such that even switching tens of amps they produce very little heat.  With the current needed for dew heaters they don't even get warm without any heatsink.  Hence no cooling required.

Initially I used the ultra cheap IRFZ44 NPN power MOSFET with electrically HOT dew heaters.  ie. the 12v (nominal) went straight to the dew heater and the -ve line was switched.  This means that great care must be taken with insulation.  Far safer and highly recommended is to switch the 12v line to the dew heater, then if it's OFF there is no shorting hazzard.  So the NPN power MOSFET was replaced with a PNP version in the 12v side.  These still have the ultra low ON resistance and don't even get warm.

To provide the 7-12v logic level required for the PNP device from the 0-5v Arduino O/P I used a simple inverting circuit using a cheap 2N7000 signal MOSFET and one resistor.

The ferrite choke and capacitor on the output stop any RFI from the fast switching.  The choke is simply a ferrite ring with ordinary hookup wire threaded through several times.  The capacitor a 100nF ceramic type.

[Gina]

I'll post the Arduino sketch later but here are a few notes on the control system...

The DHT22 is a digital humidity and temperature measuring device that connects to the Arduino with just one data line plus 5v and Gnd.  A library contains all the data reading code and the sketch contains code to calculate the dew point from the temperature and relative humidity.  Dew point is the temperature at which dew will form and the principle is to heat the telescope objective lens (or dew shield) to a few degrees above the dew point to prevent dew forming on the optics.  The required temperature can vary considerable from ambient depending on the dampness/humidity.

The DS18B20 is a Dallas 1-wire digital thermometer that measures to an accuracy of a fraction of a degree both above and below freezing.  1-wire devices actually require 2 wires but the "1-wire" refers to the number of data lines.  1-wire devices mostly have their power derived from the data line, a system called "parasitic power" though they also have a Vcc line which may be used instead.  Parasitic power slightly reduces the maximum speed but in this context is insignificant.  The DS18B20 is housed in a simple plastic transistor TO99 case with 3 wires - Gnd, Data and Vcc.  To use parasitic power the Vcc line is connected to Gnd.

In this case I'm only using one telescope and one dew heater but my triple imaging rigs use three.  The circuit for this is presented earlier in this thread but I'll post again shortly to save anyone having to wade through umpteen posts.  Only one DHT22 is needed as the temperature and humidity are much the same in all three scope loactions (they are close together).  However, separate dew heaters are used and these have separate power control circuits and need separate DS18B20 thermometer chips.  Another advantage of "1-wire" is that multiple devices may be connected in parallel, having unique ID codes which are recognised by the Dallas library and the devices are recognised as separate entities.

[Gina]

Here the circuit diagram for a triple dew heater controller.

[Ajohn]

There is some info on another way of making the heaters here

http://www.dewbuster.com/heaters-330ohm-resistors.html

I found it useful because I had no idea what sort of power is needed.  330 ohm resistors at 12 v will dissipate 0.436w of heat and take 36ma each. The only thing I would warn people about is the power rating. I used to work at a place that had rather a lot of resistors that they couldn't use as they had to move to metal film. Free to take. I gained an impression of what a 1/2 w resistor could take from these.  Later I bought some of ebay and had a shock about just how hot they got. Seems there is an industrial and a commercial rating. I would make sure they are 1/2 w and not go lower. These for instance will run at about  70C above ambient at 12v continuous

http://uk.rs-online.com/web/p/through-hole-fixed-resistors/0148382/

John

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[Gina]

I looked into that method and decided it was too much work

[Ajohn]

 I'm a heathen. The link can help to see if this could do the job. I believe there is some sort of tape available as well

http://shop.pimoroni.com/products/electric-heating-pad-10cm-x-5cm?utm_medium=cpc&utm_source=googlepla&variant=411771581&gclid=CMyEtJyF6cMCFasBwwod6FkAoQ

Next I would wonder about a metal canned power resistor fixed to some soft aluminium that can be hand bent round some of the the dew shield - it's metal so heat will spread.

Last as it's probably hard to find rather thin stainless steel wire but it might be possible to do it with mig wire. I did make a 1m foam cutter once out of that powered via a transformer with a dimmer switch on the mains side but can't remember the voltage, it wasn't that low.  It was thicker than 0.6 mm as well.

Plus I might solder some resistors together.  A metal resistor would probably be ideal for smaller bits if needed.

John

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[calli]

My incarnation of the Arduino DEW Heater. Only one channel for my MAK.

It works on 3s and 6s Lipos including a battery alarm (beeper and switch off). I choosed to just measure the difference temperature between the corrector plate and "outside" with cheap NTCs.

Carsten