Transparent window in shielded box - Page 1
Forum section: electronics / projects

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 06:16:18 pm
content: I'd like to make an electrostatically shielded enclosure with a transparent window that doesn't break the shield.Right now, I'm thinking to use a die cast zinc project box, and cut a rectangular opening to mount a piece of ITO (Indium Tin Oxide) coated glass with conductive epoxy like MG Chemicals 8331D or similar. Does that sound like it would work?Are there better options? I tried googling around for transparent conductive polymer film and saw a lot of science papers but not a lot of people selling the stuff.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 06:22:41 pm
content: The details are important: what frequency range is important, and what is the skin depth in your metallized window at the high end of that range?[edit: high end of range]

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 06:37:03 pm
content: Thinking mostly DC and 60Hz.

author: Benta
author_rank: Super Contributor
timestamp: November 12, 2023, 08:59:15 pm
content: Electrostatic is easy.Do you mean electromagnetic?

author: jbb
author_rank: Super Contributor
timestamp: November 12, 2023, 09:05:59 pm
content: So… do you need a ‘see pictures through it’ window or a ‘what colour is it’ window?If it’s the latter, maybe you could just drill an array of small holes though it?

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2023, 09:27:07 pm
content: magnetic glass would be interesting

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 09:48:08 pm
content: I mean electrostatic.Want to read an LCD through the window.To ask the same question again,Quote from: slugrustle on November 12, 2023, 06:16:18 pm

author: jbb
author_rank: Super Contributor
timestamp: November 12, 2023, 09:55:51 pm
content: Quote from: slugrustle on November 12, 2023, 09:48:08 pmI guess some transparent conductive material would be great, but might be hard to source. I don’t have any experience with it, though.If it’s hard to source something actually transparent, would a fine metal mesh be acceptable for prototyping?

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 09:57:15 pm
content: Quote from: jbb on November 12, 2023, 09:55:51 pmQuote from: slugrustle on November 12, 2023, 06:16:18 pm

author: MarkT
author_rank: Frequent Contributor
timestamp: November 12, 2023, 09:58:04 pm
content: Wire mesh in the window will work well. ITO may have high resistivity so will have large skin-depth, not what you want if fields are changing. The wire can be fairly thin - think like condensor microphone mesh which suppresses mains interference by a huge factor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2023, 09:59:31 pm
content: Yes, that will be fine. You may want to check it out after building, put some E-field inside the box and see if it's sensible outside, or vice-versa.The other option is mesh screening, which can be quite fine, fine enough it shouldn't alias with the LCD pixels; but it won't be cheap.There's also just plain old distance, if you don't need the E-field in front of the box to be small but merely at a distance, just don't worry about it at all; it's quite high frequencies where you need to worry about emissions from a hole, slot or cavity.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2023, 10:02:47 pm
content: magnetic glass is like transparent aluminum. scotty knows

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:09:28 pm
content: jbb, MarkT, T3sl4co1l,Maybe something like this for the wire mesh?https://www.mcmaster.com/9241T61-9241T611/It claims 59% open area with 0.008" wire diameter. The LCD would be 7-segment or 14-segment, so aliasing of tiny pixels is not a major concern.I'm still partial to ITO glass, in part because it would prevent dust ingress, but nice to know what all the options are, and fine screen does seem like a good one.

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 10:11:16 pm
content: You can use a thin sheet of clear Mylar behind the mesh (mesh in direct contact with the metal box) to keep dust out.(Normally, though, Mylar cannot be glued, so you might try very thin rigid polystyrene from the model railroad store.)

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2023, 10:16:37 pm
content: ok but for the original question, you want to get a conductive gasket. ITO glass conductivity is pretty poor. And you don't want to glue that if you don't have to. You can get all sorts of conductive gasket material. Then you sandwich it with a backing piece.

author: jpanhalt
author_rank: Super Contributor
timestamp: November 12, 2023, 10:21:39 pm
content: Quote from: slugrustle on November 12, 2023, 09:48:08 pm1) Why not put the LCD outside the box?2) As for "conductive epoxy," do you want an adhesive or just a very low temperature (eg., 60°C) "solder?"

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:22:35 pm
content: Quote from: coppercone2 on November 12, 2023, 10:16:37 pmIs there a reason for not wanting to glue it? I specifically mentioned electrically conductive epoxy for the glue (MG Chemicals 8331D).Conductive gasket with some little clips to apply pressure makes sense.

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:24:53 pm
content: Quote from: jpanhalt on November 12, 2023, 10:21:39 pmIf I can have all the relevant circuitry, including the LCD, inside the shielded box, then there are no questions about the shielding. If I put the LCD outside the box, I'd want to galvanically isolate any connections to it, which multiplies the number of power supplies and so on, even if i'm only isolating a single line digital interface.Low temperature solder is a good thought.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2023, 10:32:12 pm
content: The ITO coated galss is not that bad in the conductivity, definitely OK for 60 Hz and small size. Most LCDs actually use some ITO. The question may be where to get it with enough thickness and not just the very thin form to effect IR reflection.Magnetic glass is no that excotic - it is just metallic glass and not transparent anymore. You can find it in quite some anti theft stickers (older style, about 45 mm long).

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 10:34:21 pm
content: Yes, "magnetic glass" means that the metal is glassy: i.e., amorphous, instead of polycrystalline.

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 10:44:51 pm
content: I'm seeing ITO coated glass in about the right size (75mm x 25mm) with 1.1mm glass thickness and 10Ω/square resistivity on ebay.

author: BrianHG
author_rank: Super Contributor
timestamp: November 12, 2023, 10:47:17 pm
content: You might be able to use a 50% transmissible clear-back mirror.What you basically have is glass with a thin coating of aluminum or gold for gold colored mirrors.However, you will need to clear-coat the back of the mirror as it will be the exposed side and you dont want anyone who touched it to cause erosion. You will also need to mechanically connect to that mirror's outer edge. Shielding metal sponge gaskets designed for RF enclosures should do.Your display shining through the mirror will be dim. Get a display with a strong super-bright LEDs.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 12, 2023, 11:03:58 pm
content: You could do it low tech pin-hole camera style: Plenty of brightness on the LCD, a single small hole in your shielding and a rear projection screen mounted above it

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 12, 2023, 11:11:25 pm
content: Quote from: shakalnokturn on November 12, 2023, 11:03:58 pmNow we're talking. I hope the patent office is still open in your timezone. Call quickly

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2023, 11:41:53 pm
content: because gluing on a piece of glass to a chassis element is like dodgy. If you put a gasket it gets a bit of impact resistance since its compliant. I don't recommend doing that. Even picture frame don't glue glass.A home window is glued with stretchy silicone too. The electronics glue is hard.Thats my opinion anyway.Like bonded glass is usually bonded on the whole surface with transparent adhesive, or its reinforced (be it like cell phone glass 'corning' or car windshield which is multilayer)Pretty sure your ITO glass is fragile normal glass.I would love to hear from a glass expert. Its just based on my observations on how I see glass implemented.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 12, 2023, 11:45:02 pm
content: You can't really answer your question without numbers. Any display not intended for stadium use is quite small with respect to the wavelength of the power mains. There will be quite a few dB of attenuation just from the aperture. A screen with a low resistivity layer will provide even more attenuation, but only you can say what good enough is. Do you need 40 dB, 80 dB or 120 dB of attenuation?It is both easy and hard to find how much you need. Just operate your device without an enclosure in the environment that is pertinent and measure the noise. The hard part is making sure that the noise you observed is field induced since there are many sneaky conductive paths which can affect this.The pinhole camera idea, or a small film camera if your box is big enough can answer the other end of the question. With the pinhole, or no window at all what is your observed noise. The two measurements will tell you what the window attenuation has to be until you work on other noise sources.

author: pickle9000
author_rank: Super Contributor
timestamp: November 13, 2023, 12:45:13 am
content: Use an ultrasonic transducer and pickup outside the box.

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 13, 2023, 03:43:48 am
content: Quote from: coppercone2 on November 12, 2023, 11:41:53 pmThanks for explaining.Agreed that ITO glass is just normal glass with a thin layer of ITO on one side.Another issue with gluing is the difference in coefficient of thermal expansion between the enclosure and the glass.

author: slugrustle
author_rank: Frequent Contributor
timestamp: November 13, 2023, 03:44:50 am
content: Quote from: pickle9000 on November 13, 2023, 12:45:13 amVibration would be bad in this application, but along similar lines, a plastic fiber optic cable might be really nice. The TOSLINK ones aren't insanely expensive.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: November 13, 2023, 02:02:24 pm
content: Seems like you can order an ITO-coated PET sheet from Adafruit:https://www.adafruit.com/product/1309They also carry a smaller ITO-coated glass sheet. The PET sheet will likely scratch very easily, but would lend itself to gluing with conductive epoxy.For what it's worth, Keysight uses an ITO sheet over the LCDs of various spectrum and network analyzers, presumably to meet radiated emissions specs. The usual method uses a fabric-over-foam gasket to couple the ITO-coated sheet to the chassis. One of Shariar's videos may have shown this in detail, though I can't remember which one.

author: pickle9000
author_rank: Super Contributor
timestamp: November 13, 2023, 07:20:08 pm
content: Quote from: slugrustle on November 13, 2023, 03:44:50 amIf your only doing one data point any led an optical fiber and use a handheld RPM meter.