Sunday, January 25, 2009

Integrated Circuit to open the front door

I'm blessed with a home that has an automatic sliding door, and these generally come with a button that opens the door from the inside (our door is set not to open on proximity, since the entrance is unmonitored). Of course, if you have a door that has to be manually operated, you'd have to create your own opening mechanism, which I won't be writing about (at least not anytime soon). The button looks like this:

(click for larger image)

The red arrows indicate the contact points. What happens when the button is pressed, is that a contact is made and the door opens. What we want to achieve is simulate that the button has been pressed by creating the contact ourselves (no, not by creating a robot with wi-fi that presses the button on command, there is an easier way ;-)). We can do this simply by the means of a transistor and it would look like this:

This diagram is made with KTechlab (linux users), by the way. If you're a Windows user and want to simulate a circuit, try getting your hands on a student license for Multisim if your university offers this. You can also download a trial.

Come to think of it, I wasn't entirely accurate about the "simply" part. It was possible in my situation to use a (NPN) transistor, but I had to account for the following:

Let's examine the transistor I used a little closer, it has 3 terminals: Collector, Emitter and Base. As the symbol indicates, the current can flow from C to E, but not the other way around from E to C (if you don't have a clue why, that arrow is called a diode and it only allows current to pass in the direction the arrow is in; but then again you probably should know that). This means that only a Direct Current (DC) is allowed to pass! Hmm, wait a second, do I know for sure that the circuit that controls the door operates on DC? No, I don't! So, I get out my multimeter and test for it (which I had to buy for this purpose, but I didn't think it would be the last time I used it, so I decided it was worth it. Those things are cheaper than you'd expect, I got one for 8 euro at a local shop I deem expensive). As it turns out, the door switch circuit operates on AC (the voltage on the switch was about 82 Volts).

But, if I send a direct current (DC) in the right direction, the door opens anyway. Lucky me! I found this out while doing measurements with the "Diode and buzzer" setting on my multimeter. What this setting does is, that it allows a DC to pass, and when it does, the buzzer sounds to indicate that the two points are connected. (If this doesn't work for you, you need a relay instead of a transistor, which allows for AC to flow. Mirror in case of failure.) So when putting the transistor in place, I have to take notice that the DC that activates the door switch is allowed to pass.

Later on, we will see that interfacing this IC to a computer, will need some precautions that are not yet shown here.

If you want to learn more about transistors, Wikipedia is always a good place to start and Google (click for search results on "Transistor") is your best friend.


  1. I set up a webpage about the 2n3904 transistor if anyone is interested.

  2. Your blog took to me an entirely significant spot. It is a beneficial and factual article to enhance knowledge. Thanks for sharing an article like this.Online Vlsi Institutes

  3. A very delightful article that you have shared here. Your blog is a valuable and engaging article for us, and also I will share it with my companions who need this info, Mini Audio Power Amplifier Board Thankful to you for sharing an article like this.

  4. replica bags lv replica hermes d0u08h5s46 replica bags toronto best replica ysl bags visit site n3c46o4f61 replica bags manila visit this website s4f54a9d18 louis vuitton replica handbags replica bags china g0g61n3g61

  5. website here j9c33h8j49 louis vuitton replica best replica bags online 2018 replica bags for sale hermes replica v2c70m2i80 replica bags in dubai look what i found u2z51k8r30 best replica designer replica bags toronto