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Practical question: This controller will operate out in the field, at night, where dew (possibly rain too) is a reality. I need to run a few wires (up to 12 or so, maybe less) between controller and the main instrument, and the connector must be sealed against humidity, obviously. The amperage through those wires is tiny, maybe 0.1 A each, peak.

The 20x4 LCD needs to be sealed around the edge, so that humidity doesn't get inside the box.

There will be a few buttons, maybe 6 or so, on the controller, to drive the menu displayed on LCD, those need to be sealed too.

Finally, there's a DC input connector that will need some kind of rubber cap.

I've not decided yet on the box that will contain the controller. Plastic and aluminum are obvious but not exclusive choices. The total size should be about half the size of a DVD-writer unit for a PC. I'm mentioning material and size only because this may influence the choice of connectors and buttons, otherwise this is not a problem.

Any ideas how to approach this issue in a way that's reliable?

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First you should take a look at the IP Code and decide the level of protection you desire. The IP Code rates enclosures, connectors, switches etc in terms of their ability to survive in harsh environments including water. In the marine (boats) environment a rating of IP67 is often specified, but that is probably overkill for your application.

Once you know the IP rating you want you can then select a connector that meets that rating. One type of cabling you might consider is Ethernet cable if it has enough wires to meet your needs. IP67 Ethernet connectors are readily available. Here are some, but shop around as I am sure you can find much better pricing.

Here are some IP67 pushbutton switches.

As far as the LCD goes, why not just use a box with a clear plastic front and mount the LCD inside the box? For something cheap you may be able to use a standard plastic electrical box from Lowes or Home depot such as this (they come in various sizes). Instead of the standard cover plate, just cut one from clear plastic.

As far as humidity getting into the box, there is a reliable way to prevent this, but it would probably be way overkill for your application. I simply describe it here in case someone else reading this question has more stringent requirements. The box can be filled with an inert gas like nitrogen under pressure. This will prevent the ingress of moisture.

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2 ways to go here. Only thing is, I don't know any of the technical terms for any of this, so if somebody could help out...

In both cases, you will need to cover the switches and led with some flexible plastic and print some art on the plastic. That way you only need to worry about sealing the plastic (a simple rectangle) and not every miniscule component.

If this is a one-off project, you can seal all joins with silicone sealant and go home. If, on the other hand, you're setting up manufacturing you will have to design something like this http://www.crazypc.com/products/8302.html, only backwards (if that makes any sense). Basically both sides of the join will have a recess all around the perimeter, and you will put a glorified rubber band between them. I'll draw a picture later, now I just don't have time.

I did have, in my water softener, a pcb that was covered by some clear water-resistant varnish. The varnish protected the PCB 100%, but the metal housing covering one portion of the PCB was rusted solid after a year. Oops.

Re connectors, just google - there has to be an existing solution. Also, make globalspec your friend.

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    \$\begingroup\$ Silicone Rubber sealant is very useful but it is pervious to water vapor and the u=interior RH will match that externally after a while. In a stable atmosphere you have an equalisation time typically of days. \$\endgroup\$ – Russell McMahon Mar 24 '12 at 3:13
  • \$\begingroup\$ Did not know this - thank you! What alternative would you recommend? I've heard ppl using vaseline but that's a kludge. \$\endgroup\$ – Joe Stavitsky Mar 24 '12 at 5:19
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    \$\begingroup\$ Silicone Rubber is OK - you just need to be aware that it is not the perfect seal that it appears to be. There are very few better seals available that are suitable for general use. There are some very special plastics that are literally 1000's of times bettet re water permeability, but they are usually annoying to work with, at best. What upu want is a conformal coating inside any enclosure that keeps liquid water off surfaces and that has a very low %age of dissolved water. Water vapour is bearable. Silicone rubbers in intimate contact tend to do OK. \$\endgroup\$ – Russell McMahon Mar 24 '12 at 6:05
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    \$\begingroup\$ As a guide to hiw hard it is to keep water vapor out: The EVA laminating sealant used in PV panels is about 10 x less water permeable than silicon rubbers. If you laminate two sheets of glass together with a say 100mm bead of EVA all around the outside edge so that the only path into the core is theough the seam side on (ie 100mm long path with height = thickness of sealer, you can expect water valor levels inside the panel to equalise in weeks to a few months. The 20+ year lifetime of solar panels does NOT come because of water exclusion. Even many people who make panels don't know that :-). \$\endgroup\$ – Russell McMahon Mar 24 '12 at 6:08
  • \$\begingroup\$ I didn't know that about silicone rubber, and I was thinking to use it, thanks. One alleviating factor is that the controller will not be exposed continuously to humidity. Rain will be very occasional, more like an accident. Exposure to the elements will be intermittent - a few hours at night (or day, rarely), then it goes back inside the house or garage. So continuous buildup of water vapors seems unlikely. I assume silicone "glue" is similar (the kind that remains flexible after curing and can be cut with a thin strong wire), is that right? \$\endgroup\$ – Florin Andrei Mar 24 '12 at 20:48

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