Electronic Card Key Switch

15 Dec

It’s Christmas break for us students!! I know some of you guys go on vacation. Visiting relatives from different places or, others just staying home.

On my side by the way, the whole family and I are on a land trip to Zamboanga city to visit our lola (grandmother). On the way, we stayed on various hotels at different places such as Davao, Cagayan, Iligan, Pagadian.

Well as every hotel we check in. I come to notice every room has a card switch on them. I bet you’ve noticed them too, right?

Well, card switch: It’s like card slot (usually beside the room’s main door) where you place your room key or card in, to power up the whole room. Fridge, television, aircon, and most likely – room lights.

So i begin to ask myself, “How do these switches work?”

After quite a bit of day dreaming, I came to come up with an idea!

Why not, point an LED(Light emitting diode) to an LDR(Light dependent resistor) for about 5mm of space. And place the card as an obstacle that blocks the light from reaching the LDR.

Eng’g Talks: “if you’re not interested in this part, you could skip right through ahead the output :)”

Here’s the circuit:

As the light across the LDR darkens, its resistance rises therefore acquiring a larger voltage drop. As you can see a transistor is placed to switch the sensor current to a much higher current which drives the RELAY which, switches the 220v component.


As i place the card in, my desk lamp switches on ^^, after I’m done with my work I’ll just remove the card and the lamp turns off.

Sure there’s a whole bunch of card keys out there such as radio frequency card keys, magnet and chip based. This is just one of those.

Just like the ones in hotels 🙂


Posted by on December 15, 2010 in Self made projects


Tags: , , , , , , , , , ,

2 responses to “Electronic Card Key Switch

  1. asyraf

    October 2, 2011 at 4:03 pm

    hello..can you help me on this card key switch??how can we modify this thing that it can only detect certain card only??such as employee card or credit card. Can you send to me the circuit??
    my email

  2. ElectronicGirl

    May 26, 2014 at 9:14 pm

    so cool 🙂


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