Ever wonder what is inside a RFID card, or how it works?
About 6 months ago, I built a RFID door lock system for work, but the RFID cards are thicker than I wanted. I started thinking how cool it would be to have my phone open the doors. So I did some research for a RFID phone case and found out none existed at the time. I started thinking about how acetone melts rubber/plastic and what it would do to these cards. Would it still work, and could it really fit inside my phone, would it cause signal issues? Turns out it will fit, and works just as good inside the phone as outside.
I went back to manufacturing borrowed some acetone, found a glass beaker, put the card in the beaker with a inch of acetone, put the beaker in a ziplock bag, closed it up, and wrapped a rubber band around the top of the beaker. (I was told acetone evaporates quickly so I thought the bag would hold it in the beaker.) This also keeps acetone from splashing.
30 mins later of soaking, and slowly swirling the acetone around.
15 mins later
When I was pulling some of the plastic layers off I broke a wire. It took 5 mins to reattach it.
Once I removed the two screws from my iPhone 4s. All I needed to do was tape the coil and circuit board directly to the inside of the back cover.
I've been using this for over 6 months and I have never had an issue.
How It Works!
I've done very little research on RFID but I do know a little about radio frequency so I'm going to take my best guess at how these work.
1. The RFID reader sends out a 170 KHz RF(radio frequency) when the device is trying to read an ID.
2. The RF is received by the RFID cards coil which is tuned to the 170KHz signal.
3. The RF at this point is a small amount of DC power, just enough to power the circuit board, and the board sends the ID back to the reader.
I'm not sure if the circuit board uses the coil as a antenna, or has a small one inside or on it somewhere, but this is why RFID cards do not use batteries, because the power comes from the card reader.
I'm not sure which came first, RFID, or wireless phone chargers but both use the same idea. Inductive power transfer.