Building a small Faraday Cage box

[Roger Feeley]

My daughter is trying to get her boyfriend to unplug more. I thought about making her a sort of jewelry box that would act as a faraday cage where they can sequester their phones. Yes, they could just turn them off our put them in airplane mode but I'm thinking the symbolism of the box might be a good thing. You just toss the phones in a box.

Also, young people are having parties where everybody agrees to unplug. A box like this might be nice there too.

So I have some questions:
1. Does the 'seal' have to be perfect. What about the gap between lid and box.
2. For instance, could I just create a laminate of aluminum foil between wood and make a box? Or maybe make a box, line it with foil, and then add another layer of wood.
3. Could I just gild the inside of the box? Is there a minimum amount of material.
4. Does the box have to be grounded (deal breaker).
5. What does a phone do when it can't make contact. Will it go crazy trying to contact the cell network and burn through the battery?

thanks,

[Matt Meiser]

On #5, that's been my experience. My phone battery normally has no problem making it through a long day or more but if I'm not getting good service, it can run down by mid afternoon even without making calls.

What about building the box so it can double as a valet, but if you close the lid, it blocks the signal? Put a hidden power strip to plug in chargers with small slots for the just the business end of the cords to stick out into the main compartment of the box. That also gives you a convenient way to ground it if needed (which I think it is.) That also solves #5, because now the phones will charge as well.

[Mike Henderson]

I'm not an expert on Faraday cages, but this is what I know:


My daughter is trying to get her boyfriend to unplug more. I thought about making her a sort of jewelry box that would act as a faraday cage where they can sequester their phones. Yes, they could just turn them off our put them in airplane mode but I'm thinking the symbolism of the box might be a good thing. You just toss the phones in a box.

Also, young people are having parties where everybody agrees to unplug. A box like this might be nice there too.

So I have some questions:
1. Does the 'seal' have to be perfect. What about the gap between lid and box.
Any opening in the cage has to be significantly smaller than the wavelength of the frequency you want to block. Note how cell phones work inside an airplane because of the windows (big compared to the wavelength). So I'd put a metal gasket between the top and bottom.
2. For instance, could I just create a laminate of aluminum foil between wood and make a box? Or maybe make a box, line it with foil, and then add another layer of wood.
That should work, as long as you don't introduce any holes.
3. Could I just gild the inside of the box? Is there a minimum amount of material.
As far as I know, there's no minimum to the amount of material, but the material has to be a good conductor. You'd have to be sure that gilding conducted across the surface. Note that static bags that electronics are stored in are Faraday cages.
4. Does the box have to be grounded (deal breaker).
To act as a Faraday cage, the box does not have to be grounded (see previous note about static bags). The requirements are that the walls are good conductors, that the walls are electrically connected together, and there's no holes that are close to the wavelength of the frequency to be blocked.
5. What does a phone do when it can't make contact. Will it go crazy trying to contact the cell network and burn through the battery?
I can't answer this one.

thanks,

[Alan Lightstone]

It's my experience that #5 is definitely true.

[Kevin W Johnson]

#5, As Matt stated, the batteries will drain quite fast as the phone goes into "search" mode trying to find a cell signal.

Whether or not you need to add a charging feature to the box depends on how long the phones will be in there at a given time. But it certainly would be a good idea.

[Alan Lightstone]

Or they could turn them off while they are in there. Afteer all, it's not like they serve any purpose besides ballast while in a Faraday cage.

[Jerome Stanek]

do people even know how to turn off a phone

[Jim Matthews]

Once you have your prototype, please make a version large enough to fit over a movie theater.

[Jay Rasmussen]

She needs to find a new boyfriend!

[Jim N Burke]

Or you could just say that it is a Faraday cage, BUT they have to turn off the phones for it to work properly!
Seriously, number 5 is likely true but they probably won't leave the phones in full time. You can also test the concept with a cardboard box wrapped in aluminumfoil.
Jim

[Myk Rian]

Build a nice wood box and line it with foam to absorb sound.
That way, the phones won't discharge while looking for a signal, and nobody will hear the ring tones.

[David Nelson1]

Or you could just say that it is a Faraday cage, BUT they have to turn off the phones for it to work properly!
Seriously, number 5 is likely true but they probably won't leave the phones in full time. You can also test the concept with a cardboard box wrapped in aluminumfoil.
Jim

Ding Ding Ding

[David Nelson1]

Copper mesh screening is what you want. The holes are very small say the size of a ball point pen tip and thick. Very expensive. Just turn the phones off!

[Josh Rudolph]

As you lose "towers" or reception on a cell phone, the software adjust the hardware to utilize amplifiers in searching for a signal. These amplifiers drain the battery at a higher rate. Cell phones can last for a long period of time with great reception and very limited amount of time when reception is spotty.

The use of the Faraday cage for the phones could end up harming them in the long run. Essentially it will heat the phones due to running them harder looking for a signal and then they will be in a sealed box.

Using mesh does not always "kill" the signal. A mesh size of λ/20 (for cell phones assume λ or wavelength of 900MHz to be 33.3cm and λ/20 to be 1.67cm) of the operating frequency of the phone would only attenuate the signal about 50% meaning they will still have reception. A mesh size much smaller than λ/20 is desired. If you are really wanting to build a Faraday cage for cell phones, your best bet to have a solid copper wall enclosure with all of your seams taped with copper tape and a knife edge lid that sits inside the copper box making contact on the sides and the top with a gasket. RF signals just have a way of wiggling themselves out of what you think is a sealed space.

As others have said...best option is to just turn them off.

[Gary Herrmann]

Aluminum foil could work, but I'm not sure. It would have to be as continuous as possible and not broken by the wood substrate. Frequencies of the phones would dictate the size of any gaps in the box. Maybe a double layer box where you're offsetting joints, wood and aluminum would work.

[Roger Feeley]

Thanks to all for responding. I knew that you guys have a great breadth of general knowledge and I would get good answers. My takeaway is that it's a bad idea.

I still like the symbolism of the box but if it's going to drain down the batteries, no one would use it.

[Jacob Reverb]

do people even know how to turn off a phone

I thought that required a surgical procedure.