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Introduction
Introduction
A contactless payment card like any modern Visa or Mastercard is much more compact than it seems on first sight. The electronics of the card are all hidden behind the seven electrical contacts you can see in the picture. The tiny electronic chip is about 1.5 x 1.5 mm. So if you remove all the unnecessary plastic, you can integrate a payment card into almost any object!
There's one catch: to interface with the card, you need an antenna that can be used for wireless communication and power transmission and NFC frequencies. This project was about embedding the card chip and the antenna into my watch
Extracting the chip from the card
Extracting the chip from the card
The payment card I used for this project could not be dissolved in acetone. So instead, I used a hot air gun to extract the chip. After melting the plastic and bending the card, the panel with the electrical contacts falls right out of the card.
The actual logic is only the tiny square on the back of the small panel. However, we will keep most of the panel as it contains electrical contacts that the antenna can later be soldered to
Making the coil
Making the coil
I lasercut a simple jig to wind the coil to the right size, so it would fit neatly in the bezel of the watch. Then I soldered 50 cm of thin insulated copper wire to the two contacts on the back of the panel with electrical contacts. The copper wire was harvested from an audio transformer.
The antenna has to be rather small as the watch housing is approximately 29 x 29 mm on the inside. My first antenna has 3 loops, but due to the small size, this antenna was not sensitive enough to work properly with all payment terminals. The second iteration has 5 loops, which more than doubles the sensitivity and inductance of the antenna.
Embedding the coil in the watch
Embedding the coil in the watch
I cut a groove in the inside of the housing of the watch to hide the antenna coil. Next, I trimmed the panel with electrical contacts, as the panel was still too large to hide it in the watch. After cutting of the excess of the panel, I could hide it next to the coin cell battery.
Then, it is a matter of placing the coil in the housing and closing the watch back up. The water-tightness remains as the watch is identical from the outside as before the modifications.
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