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Topic: How to isolate a circuit from water  (Read 494 times) previous topic - next topic

123Splat


Grumpy_Mike

Why not put the PCB further up his arm , so it doesn't get wet .
Because when he raises his hands water will drip down his sleeves wetting the PCB.

So now we are on page 2 we finally get a full description of this former X-Y problem.

You can not use resin for this. It is a very difficult task, I would hesitate to take on a project like this. It will end in tears.

Try and remove the switches from the hand and have some other way of control like an under arm switch, or magnets and reed switches. The so called water proof LED strips are not water proof enough. To find out how water proof something is you need a measure known as an IP rating, led-ip-ratings-led-flex-strip

tinman13kup

I think I would look at some fiber optics for the lighting
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

JohnRob

One important factor is the expected life or reliability.   Are you trying to make a product or just a hobby project?

If a hobby project, you need to seal it well enough it will last a reasonable amount of time and it doesn't fail shortly after starting to use it.

Not sure what switch you have in mind so I'll put that aside for now.  The LED can be purchased as "waterproof" in eBay.  I would expect these to be at most water resistance but will likely work for some time.

For the PCB.  One of the issues with conformal coatings is they don't cover sharp edges very well.  And can be this in areas allowing moisture to eventually cheep in.

I would suggest a piece of heatshrink and an electronics grade silicone RTV.   For hobby use I know you can't use the type that emits acetic acid when curing.  You might be able to use the newer grades that don't smell as bad like GE Silicone II.  I'm not sure what their curing byproducts are.

You should use an electronics grade (i.e. non corroding) of RTV.   You can find them on Amazon, Digikey and likely other outlets.

For a trial you also might try hot melt glue.  If you use the silicone wire I mention below you will get much better adhesion from the potting to the wire if you use a silicone sealant (aka potting compound).

The procedure I've used:

  • Coat the board and wires with sealant, lots of sealant, you want not to have any air trapped on the board
  • slide the heatshring over the board and sealant
  • Shrink the heatshring
  • allow to cure

A suggestion on wiring.  If you want to be able to replace the LED and/or switch after the PCB is potted, I would simply run the wires up the arm to a relatively dry area and make the connections there.

Another word on wiring.  I've recently found some neat very flexible silicone wire.  It may be useful for your project.

ebay silicone wire
There are likely other vendors, but I only have wire from this vendor and am very happy with it.


Again if you are trying to design a saleable product none of the above will be adequate if you expect any reasonable kind of life.

Good luck.

valerio_sperati

First of all, thank to everybody. You are giving me a lot of interesting feedbacks.

I summarise here some answers to your posts:
  • about removing the pushbuttons: the proposed "reed switches" seem a good idea, I did not know them, I'll try them.
  • about the product: this is right now an hobby project but... the artist should be quite sure it works during its performance, so I need to be reasonably sure it does not fail. (If it works for a couple of performances, we will think about professional engeneering of it)
  • about fiber optics: I need a very bright light, I don't think fiber optic is fit for this.



After reading all your advices, I'm quite sure I need to build a water-proof protection box for the PCB (and Lipo!).
About leds, switches, and wires, I think I can isolate them using the proposed Electronic Grade RTV Silicone, the heatshrink tube, and the silicone wires (for optimal adhesion).

Thanks for sharing knowledge: I learned a lot of new terms: IP rating, Electronic Grade RTV, and X-Y problem definition.
I hope I'll manage to build a working prototype, then I'll continue to post here the result for new advices and comments.

Valerio
 


Paulcs

I am sure that you already tried -- just put a clear surgical glove over the hand?  If the intent is to be invisible, water-proofing the components (as discussed above) and then using theatrical makeup to hide anything, is an option.  The surface mount LEDs would only be 3 tiny dots. 

valerio_sperati

Yep, I tried surgical gloves  :)
In theory they are a good solution, but  the problem is that they are extremely fragile: they often tear when touch something different from hand skin. This is because they are really tight and thin. I cannot use large size gloves, as the performer needs a great sensitivity.   

TomGeorge

Hi,
Quote
I cannot use large size gloves, as the performer needs a great sensitivity.   
Yes it would spoil the illusion if the performance was done in bright yellow dishwashing gloves.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

TomGeorge

#23
Sep 22, 2017, 09:13 pm Last Edit: Sep 22, 2017, 09:15 pm by TomGeorge
Hi,
If it is possible to waterproof the LEDs, why cant the controller be mounted on a waist band with wires up the arms, and some switches mounted say on the sides of boots or shoes.

The you just do a Dorothy from the Wizard of OZ, and click you heels or toes together.

@Grumpy_Mike, this sparked the  idea.

Quote
Try and remove the switches from the hand and have some other way of control like an under arm switch, or magnets and reed switches. 
Thanks Mike. :o


Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Paul_KD7HB

I think is may be possible to do all your hand sensors and LEDs using optical fibers. The LED lights will certainly show light at the end of fibers. The switch could be two adjacent fibers. Light in one and the adjacent fiber connected to a light sensor. A finger or other object would reflect the light into the second fiber.

Paul

valerio_sperati

In general I need to keep the wires from PCB to LED and switches as short as possible. This is because long wires (for example wires going from hand to waist) will probably interfere with performer's movements (performers are going to panic in these cases  :)  ). This is why I think that, for the performer comfort, the best assembly of PCB is on wrist. As I said, the performer always wears a wristband preventing soapy water from running along the arm; I could exploit this sweatband for PCB: it will be wet, but it will not dipped in water.      

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