Saskatoon TechWorks Forum

Ahoy everyone. So this is something I've looked into for about the past year. My plan was to begin the project immediately after Christmas, but since the hackspace got started, I figured that we as a group could come up with a better design than my own.

My initial desire for this project was to implement subcutaneous RFID implants for me and close family who may wish to come visit my house. Through a veterinarian friend I got my hands on the proper RFID chip and equipment to inject myself. However, this plan has now been vetoed by my fiancée. So instead I decided I wanted an RFID tag that could be read as soon as I reached the top of the stairs to my house. I don't like the idea of having to swipe anything (what if my hands are full of groceries?). This would mean the reader should be capable of reading a tag from a distance of about 3 feet, mayyyybe 4 feet. This introduces us to our first problem:

1.) RFID tags are balls for being detected at long distances. At DEFCON 18 this year, I got to witness a demonstration of a high gain, high power antennae that could read a conference halls worth of RFID tags (the ones that are printed on stickers and are in long narrow strips about 2 inches long). However, the antennae's were HUGE, and the power required as sick. Why do you think stores use such large theft detectors at the entrance and exist to their stores? As far as I know, this is because the large devices act as antennae's to scan everything you're carrying. This leads us to two possible solutions:

1a.) Use a high gain antennae, possibly embedded into the siding of the house or railing around my step? This seems rather implausible.

1b.) Use something other than RFID. I really like the idea of RFID due to the abundance of low cost parts and extensive knowledge base on the internet regarding this topic. Does anyone know of any other solutions (other than using active tags)? I'd love to hear them! I will resort to an active tag if I have to, I just would prefer not to go that route. My original plan was to use my arduino in combination with a parrallax RFID antennae (http://www.instructables.com/id/AVRArdu ... Code-in-C/). There is lots of code for this and would be trivial to implement.

2.) What kind of door locking mechanism do I use? Hackaday had a keypad doorlock like the kind you can buy from home depot (there is a key pad above the handle that you enter a code into, and it unlocks the door). You can take the back panel off and wire in your own mechanism for triggering the door lock. My mechanism would be the arduino triggering this door lock. However,this would look rather ugly I believe. I would have to likely mount it on the door, but then how would it be powered? A very low power wireless chip (XBee?) could maybe run off a battery and listen for a signal sent from the arduino, but again, what if the battery dies while I'm away? Another solution exists:

2a.) An electronic door strike. This type of lock has a chunk of metal that falls into a hole cut in the door itself and latches. When power is applied, the lock is retracted to unlock the door. This is an elegant solution as it doesn't depend on the power being on to lock the door. Also, the arduino system could reside in or mounted on the wall near the door frame and control this mechanism from there. Any time someone approaches the door within 4 feet, the door would unlock, meaning you wouldn't have to rely on locking or unlocking your doors ever!

I have links to a lot of the equipment that I had in mind and can post them if necessary. I want to hear your ideas! This project is purely for my house but could be adapted for most other homes if need be. Hopefully it can act as a prototype for the security at the hackspace itself. The hackspace will have different requirements, but I think that it would in effect be similar.

Regarding the 3-4 foot distance you would like the tag to work from, are you sure you need that much range? You're going to need to reach out and turn the door knob anyways, so why not just have the sensor near the door knob? Of course, that assumes you have the tag in your hand (or maybe on your wrist, like a bracelet or watch), but would hopefully be close enough for a passive tag to work.

Regarding the Arduino, you certainly don't need to mount a big circuit board on the door to perform such a simple task.

If you use an existing electronic door lock and remove the logic board, there will be plenty of room to insert your own. Atmel makes low-power AVRs that are 8-pin SOIC (about the size of your pinky nail) and only cost a dollar or two. I have several to spare. You can program them over serial and they even have a built-in 32kHz oscillator. They also have an I2C interface, so you should be able to talk to most digital radio transceivers.

The real problem is, what sort of wireless tech do you use?

Passive tags of any sort will require that your lock provide a lot of power. Big antennas and high power make up for the fact that the tag isn't doing any work; it's just reflecting and interfering with the transmitted signal. Also, distance plays a big role. The signal power drops off at 1/r^3 as the distance increases, so a few inches is easy, but a few feet needs considerably more power. Given that, passive likely won't be feasible, unless you don't mind waving your keys or wallet near the lock (like we do in Innovation Place).

Active keyfobs can handle much larger ranges, at the expense of having batteries to replace. I don't know of any pre-fab solutions, but FM transmitters and receivers can be had small, cheap, and low in power. XBee may be easy, but it's likely overkill; the pieces are large, expensive, and consume a relatively large amount of power for your application. If you can design your own system, there are a couple things you can do to make it low power. First, use as low of a frequency as is practical; lower speed oscillations mean less power used in the chip. Second, have the lock (which has bigger batteries) send a signal to the keyfob to wake it up, then have the keyfob respond. It takes much less power to listen for a signal than it does to constantly send one out. This way, the keyfob should last over a year on a single button cell battery.

Lastly, all electronic locks will eventually fail; dead batteries are a part of life. Every electronic lock still comes with a regular pin tumbler cylinder so that you can open the door the old-fashioned way. No matter what you use, I would do the same: make sure you have a way in if the power fails. Keep a key on your keyring, even if you only need it for emergencies.

As for wireless tech, what did people think of my bluetooth idea? Have a little bluetooth module that unlocks your door when you come within pairing range? Especially on your android phone, you can use that program of which the name eludes me which allows you to pair with certain devices only when your phone is in a certain GPS area

i think bluetooth is a fantastic idea, unless you forget your phone...
but seriously, blue tooth is a good idea. with pairing and 'active' communication you can also add a a security code or pin. passive RFID's are REALLY easy to spoof (they can be brute forced). RFID's are only secure as long as people don't know you have em

How far away will your bluetooth link?
My old car security system had a function that would unlock the doors as soon as you got close. Unfortunately "close" was about 15-20 feet, which meant if I left my keys in the front room the car would unlock!

FingerTech wrote:How far away will your bluetooth link?
My old car security system had a function that would unlock the doors as soon as you got close. Unfortunately "close" was about 15-20 feet, which meant if I left my keys in the front room the car would unlock!

I know exactly what you mean but I don't think that would be as much of an issue in this case. In this case you can control both sides and tune them to only trigger when the rf levels are just right. Also the phone is going to be pretty reliable in it's rf output whereas a car remote powered by a CR30xx or AAA fluctuates it's output pretty badly over the batteries.

Personally I hate rfid, 99% of the time it can be replaced by QR codes or another better radio technology, about the only place it belongs in my opinion is in medical applications. If you aren't going to implant the the tag I would just use the phone. Why have yet another radio/antenna on you. And if the idea is to make it easier to get in the door with your hands full go with the electronic door strike that way you can pull the door open with as little as one finger. Or go all the way and add a motor onto a door closer arm

For what it's worth, a friend of mine had his bluetooth phone pair with the hands-free unit in his car... which was 2 blocks away at the time.

with an auto pair it is probably a good idea to have some sort of button to actually unlock the door. also, there should be some way to monitor signal quality/strength from the bluetooth device. the Easiest way i can think of without extra/expensive hardware is to try and send lots of information VERY quickly, more or less hoping that at distance the SNR will make the transmission not work. once you are nice and close, the SNR is good enough that the highspeed communication Works, and thus opening the door.

Those near field communicators from google look pretty cool (in the new nexus S)

I think my plan was to sew or insert the RFID tags for my door into my jackets, jeans, or wallet such that I didn't have to consciously think about them or have to remember to bring them ever.

I agree there are problems with trying to do passive at large distances. My other alternative was to mount a box on my railing containing the antennae. The railing is about waist height, so I could easily just lean with my hip towards it, and if its in my wallet, it could read it and unlock the door that way, but its a less elegant solution.

I'm betting it would be possible to use active tags with a software solution for the range issue. i.e. as soon as it detects a tag it attempts to determine the range or distance? Maybe not though? Dunno.