Let’s have a chat about our dorm key cards. 

My key card and I? Well, we’ve had our fair share of conflicts. In just over a month, I think I’ve set some sort of record for how many times I’ve locked myself out. In fact, I shamefully must admit that I even accomplished the rare feat of locking myself out not once, but twice on the day of my first midterm. And don’t even get me started on the amount of times I’ve come back from class, seeking the solace of my bed, only to find that my card has been mysteriously deactivated. Safe to say, me and my key card are not off to a great start. So, in this article, let’s dive into the malicious inner workings of key cards to understand the reasons behind their deactivation and how we can prevent it. As for the issue of locking our key cards in our rooms though, I think this article is a sign we should buy some lanyards before we lose our sanity.

Our dorm key cards work much like debit cards, with a magnetic stripe called a magstripe. The single black stripe on our key cards has up to three magnetic tracks, composed of tiny iron-based “mini magnets” about 20 millionths of an inch long. Picture each mini magnet as a simple bar magnet with a north and south end. To encode the magstripe, each tiny bar magnet is magnetized in a particular north or south direction by a device that produces a strong magnetic field. This combination of magnetic directions is unique to your card, like a magnetic fingerprint. The result? A quick slip in the key card reader is enough to authorize you and grant access to your room!

The functional component of the encoding device is a solenoid. Simply put, a solenoid is a wire tightly coiled into a helix shape. When a current passes through the wire, it produces a magnetic field by converting electric energy to magnetic energy. To strengthen the magnetic field, iron cores are usually added within the helix. When the key card passes over the encoder, it is activated. The magnetic fields produced by the solenoid write the required data to some or all of the magnetic tracks.

Then, when we insert our cards into the card reader, the back and forth movement of the magnetic stripe through the “reader head” induces a voltage in the reading device’s coils. Just as electricity can make magnets, magnets can create electricity too! The specific induced voltage is electronically recorded by the reader. The reader’s processor can uniquely identify the card based on the voltage, and then authenticates the card and opens the door to your room! 

But now, the question we often find ourselves asking: why don’t our key cards ever seem to work? As some self-reflection might reveal, we put our key cards through quite the abuse. Violently pushing them in the card reader, dropping them in our bags, shoving them in our wallets, and, in one rather unfortunate incident: spilling half a bottle of nail polish on the card (even I don’t know how I managed that one). Over time, these magnetic particles can be scratched and the card itself can get worn down. But here’s the punchline: the most common culprit of deactivation is magnetism itself.  Funny enough, the same thing that powers our cards has the ability to render them useless. A magnetic field, if applied close enough to the key card’s magnetic stripe, can rearrange the magnetized iron particles and demagnetize the card. Basically, the external magnet induces the “tiny magnets” the same way a solenoid would. The tiny magnets align with the direction of the external magnet, and the key card’s magnetic fingerprint is erased. Cellphones, credit cards, and even magnetic handbag clasps are all suspects.

Now every time you use your room card, you can appreciate the army of tiny magnets that work to produce that familiar green light. At the very least, let this article serve as a PSA to never put your key card in your phone case or with your credit card. 

By the way, if you ever see someone in a wild frenzy, begging for a temporary key at 8:55 AM because she managed to lock herself out right after a shower (and before a 9AM chemistry lecture) – that’d be me! Feel free to say hello; I’d be thrilled to chat more about the magnetic escapades of our beloved key cards (after my mad dash to the Med Sci lecture hall).