BMA Analog Access Control Card

Some place in town used to use these access cards. Eventually, some cards and readers made it from the people decomissioning the office to the hackerspace. When the hackerspace moved, it was being thrown out and ended up in my car. Two moves later, I suspect these cards are the only thing left, and on the way to being tossed out of the garage, I take them apart.

These cards have four filters or capacitors connecting different coils of wire. There do not appear to be any digital components, so I suspect these badges either all act like a shared key, or else the combination of filters encodes a pattern of frequencies that is unique to the card.