Shielded card case

This looks like a simple card case: Take a rectangular piece of leather, fold it once, and sew two sides closed. However, there is a special feature here.

The purpose of it is to hold my bus pass, as shown here. I had been carrying my bus pass in the same case as my employee ID and access card. However, the access card, which lets me into secure areas at my workspace, managed to demagnetize and wipe out all the data on the bus pass. I was able to replace it without hassle or financial loss (Metro Transit was very nice that way), but I did not want to make a habit of this. I tried carrying the pass in a pocket, but:

  1. The pockets are overcrowded anyway.
  2. They might be occupied by other electromagnetic devices (e.g. cell phone, Blackberry).
  3. This disturbed my personal organization system.

So I took two scraps of leather and some aluminum foil. I glued the foil between the two leather pieces:

After the glue dried I cut out a rectangular piece of the sandwich, folded it over, and sewed up two sides as described above. This card case will fit into the case that holds my ID and access card, so I can carry the bus pass like I did before. However, the foil layer acts as a shield (a Faraday cage, protecting the bus pass from the electromagnetic field of the access card.

This use of aluminum foil is well known. Here the leather protects the foil from the normal wear and tear of daily life and so makes it practical. It looks a lot better than other options, e.g. duct tape.

A couple notes:

  1. The whip stitch on the sides and the edge of the opening prevents the edges of the foil-leather sandwich from fraying. With a single piece of leather I might use just a simple stitch on the sides and nothing on the edges of the opening.
  2. There is no need to be fussy about the type of glue. Once the sewing is done the foil is not going anywhere.

I may use a similar technique to RFID-proof a wallet. I haven’t yet decided if I am paranoid enough for that yet.

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