Using a Door Handle Conductivity to Detect Intruders

Sometimes the simplest projects can be quite interesting, provided they’re well documented. We hope that the Hackaday readers also think that the door sensor that [Alexander] developed falls into this category. Instead of using common methods such as a magnet + reed switch, he decided to use the strike plate and door conductivity to detect someone walking in. The setup he put together includes an Arduino, a PowerSwitch Tail (a power cord that switches 120vac with a dc control voltage of 3-12vdc), a battery pack made of 8 AA batteries and two crocodile clips for door connections.

Most new hobbyists would have stopped there, but [Alexander] checked his platform’s power consumption and continued his work to decrease it. He therefore put the microcontroller in power-down mode by default and uses an AVR external interrupt to wake it up. In case beginners can’t understand [Alexander]’s code, he actually put a nice flow diagram on his website. Embedded after the break is a video of the system working.


26 thoughts on “Using a Door Handle Conductivity to Detect Intruders

  1. Interesting idea.

    How loose is the door handle mechanism inside the strike plate I wonder? I can’t imagine using the continuity between the door’s strike plate and handle to be very reliable. A change in pressure on one side of the door could cause it to move slightly and break contact with the strike plate. If this isn’t the case with the door in question, then that’s a non-issue.

    A bigger concern would be that it could be easily defeated with a bit of wire between the strike plate and handle (assuming the handle has continuity from one side of the door to the other). This is generally why alarm systems have a resistor in series with the sensor’s signal. The resistor then forms part of a voltage divider. If the sensor wire is tampered with, the input falls outside the expected range and an alarm is triggered.

    Nice work on reducing the Arduino’s current draw though. I’d switch over to something like an ATtiny13A and run it directly from two or three AA batteries, bypassing the need for a regulator. Make sure to disable ADC conversions and the brown-out detector to further reduce power consumption. Also drop the clock speed from 16MHz down to 1MHz (or slower). You should be able to hit 2uA in power down sleep mode (or so says the datasheet).

    1. If the door has a good fit, or has some soft insulation strips, the connection will be good.

      In a house with a little draft and doors with bad contact between the strike plate and lock the doors will rattle in the wind, very annoying.

      Making a connection between the handle and plate from the outside would mean the door opens outward, otherways the plate would not be accessible.

    1. I normally don’t go down that route, but in this case…. yeah. Just a little overkill. lol

      I was actually expecting him to use the capacitive sense libraries to sense someone touching the door handle.

      And why would his strike plate be connected to ground?

      1. My intent in using the arduino is that I’d like to (at some point) transmit across the room – the light was just a proof of concept. I have about a billion cheapo RF transceivers, and I’ve been just __itching__ to learn a bit about RF/wireless communication. The possibilities are neither finite, nor bounded.

        @fraganator The handle/bolt/strike plate are actually quite well manufactured – they fit together quite snugly! Thanks for sharing your knowledge of alarm systems, that’s actually quite clever.

        @justice099 I have absolutely no idea why the strike plate is grounded – it just seems like (yet another) quirk in this building’s construction. Either way, a good ground would make capacitive sensing a bit challenging.

        Funny thing, I never had any SPECIFIC intentions to use this as a security mechanism, that’s just one of many uses. That said, I can assure you that if I did try to use it as such, my roommates will negate any possible security benefits :)

      2. Probably to reduce the chance of static shock. Instead of building up in your body till it discharges at a high level, you’re constantly discharging it to ground when you walk in and out the door when it’s at a very low level. I guess whoever had it constructed had that as a design requirement.

        1. That would actually do the opposite of what you think. A direct ground with no resistance in between would shock you even more. In fact any conductive surface will.

          And if it is a true ground routing back to the service box, I would think that would be dangerous because there may be a difference of potential between that ground and say the floor. Any later construction may break that connection and now you are standing in between two separated earth grounds.

        1. Except that the Arduino and the 8-pack of batteries aren’t the most reliable things in the world…. And “no duct taped magnets” – well there’s a lot of other taped things instead. :-)

          Still a cool idea though.

  2. I did the same using the capsense library, with a small box that would count the number of times the doornob was touched, unfortunately all the doornobs here are brass so it didn’t work very well, otherwise I’d have posted it :/

    1. Hm? Why wouldn’t capsense work with brass knobs? It’s a metal just like steel or copper. If they was made out of plastic os glassfibre I’d understand – but brass?

  3. The door frame is most likely grounded because of metal studs in the wall, and there’s electrical conduit secured to them. I don’t deal directly with the door or frame installation, but I can’t recall any electrical code that requires the frame to be bonded to earth ground.

    Obviously, this method isn’t going to work in any construction that uses wood instead of metal studs.

    Regardless of the quality of the strike plate and other door hardware, some doors have have a loose fit of the strike in the strike plate, and others will be snug. It all depends on how well the door frame was installed. The slightest tweaking can make the difference between a door that swings shuts and latches, and one that has to have an extra push to latch.

  4. It’s a nice project. Thing is most burglars never would touch the handle, a door like that is easily opened using a crowbar. Ask the cops in your district what the most common way is to break-in.

    It’s not as if burglars actually buy DX lockpicking tools and go out wearing gloves and ski masks…. At least not for some college kids dorm room ;) (no offense)

    *and remember kids, putting high voltage on door knobs is a criminal offense…

      1. I’m trying it on a breadboard as per your fritzing and using the sense wire to contact positive and negative to operate it -no pushbutton. The problem seems to be not the wiring but the interupt code . It gets the first state when it resets but does not change after that no matter where I put the sense wire . I have Duemilanove boards and I have not noticed a polyfuse .Does your interupt code work on a 328P chip? Have you tried it at all . Are there perhaps some timer or register changes required for the 328P

        1. Duemilanove boards absolutely SHOULD have a polyfuse, so that shorting 5v ➡ ground cuts power to the board. Does shorting 5v ➡ the strike plate on your door have the same effect?

          Both the Leonardo and the Duemilanove have pin 2 mapped to an interrupt-supporting µc pin, and unless you’re writing assembly instead of C, you shouldn’t need to make any changes.

          what exactly is your sense wire connected to? What happens when you replace the resistors with jumpers?

  5. Oh and I see there are a lot of nutters on here .Silly unnecessary posts . Ignore them . When you get to radio control you will find the 5 volt modules generally have a very limited working range and ability to penetrate walls etc. Try the 12 Volt versions and the lower the frequency the better .Study the antenna needed and get it right . Measure from the solder joint, or even add the lead in track , to the tip of the antenna .That can make all the difference .I get 1 km on mine at 300Mhz

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