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I'm wanting to make a type of electronic "trip wire".

My basic idea is to have like a laser and basically when the laser is interrupted then it sends some signal to a microprocessor.

I'm not tied to a laser but it was the first thing that comes to mind. The attributes it needs to have is it needs to be only like a "wire"(as in, it doesn't detect motion anywhere but head on) and it needs to be low power(like run for a few days at least off of 2 AAs). Also, an actual wire is out of the question because it needs to be capable of being "reset" after a certain time out(of say 1 second or so) The range it needs to work with is only 5 or 10 feet though. Also, it's not possible to have two parts to this device like a typical laser-photodiode setup. This needs to be contained all in one (preferably small) package.

What is my best bet with this? I was thinking maybe super-sonic sound like this but I think there would probably be a better way.

So basically what I have in mind for the device is like when you first set it up, you "calibrate" it for a certain distance. So for instance if there is a wall five feet in front of it, then this device would work if there was anything that came head-on with it in 5 feet. And etc.

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    \$\begingroup\$ What should trip the wire, and what shouldn't? Eg: fog, rain, pant leg, wheel hub? Also, can you install a mirror/shiny thing on the other side? \$\endgroup\$ – tyblu Dec 28 '10 at 7:50
  • \$\begingroup\$ @tybl Well I would want it to be half resistant to things like fog and rain. But basically I'd want for say a leg or animal or something like that to trigger it. And no a mirror can't be installed. It has to all be contained in one box basically. \$\endgroup\$ – Earlz Dec 28 '10 at 9:36
  • \$\begingroup\$ Maybe something like a ultra sonic sonar may work better in the foggy or smokey conditions. \$\endgroup\$ – jsolarski Dec 28 '10 at 11:54
  • \$\begingroup\$ Nah, a laser is a good idea. Will post about it tomorrow if someone else doesn't. \$\endgroup\$ – tyblu Dec 28 '10 at 13:01
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    \$\begingroup\$ Mirrors are a pain to install; you'd use a retroreflector instead of them. \$\endgroup\$ – Nick T Dec 28 '10 at 15:22
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There are multiple options for such a thing.

  1. Ultra sonic sonar
  2. PIR sensor, used in security systems

Your best bet I think is to use a PIR sensor like this one.

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Your requirements:

  • line-of-sight activation: this means focusing lenses & apertures for LEDs, or a laser.
  • self-contained unit: this means it must register a reflection from a variety of surfaces.
  • low power: doesn't really limit implementations, but puts some design constraints on them.

There are two options:

  1. Infrared emitter and detector package, like the one linked in Matt's answer, but with a focusing lens or aperture or a small viewing angle.
    • Focusing lens, aperture, or small viewing angle is required to maintain directionality.
    • Focusing lens or aperture would be a pain in the butt, and could only be tuned for certain distance ranges.
    • Viewing angles as low as 6o are available. Geometry will tell you what kind of directionality this gives you at a distance.

  2. Laser and photodiode: Lasers are available with focusing lens and aperture for less than $5 each; photodiodes are about the same. I would use a visible one to make aiming it easier (but require bandpass filtering around carrier frequency to filter out ambient), but it's up to you: there are IR and (expensive!) UV laser diodes. It may take a bit of experimentation to develop an algorithm that filters out ambient effectively and detects weak reflections. This guy did something like it. Note that some surfaces will simply gobble up a visible beam (eg. flat black).

For low power operation, use sleep modes, pull-up PMOS FETs for always-on devices, including voltage divider branches (stop current drain when controller is off), and activate the sensor intermittently. It doesn't sound like it needs to capture any fast single-shot events; going with 20km/h and 10cm objects, a 0o (perfect line) detector would get 18ms to react, or just over 55Hz. You could wake-up every 10-15ms, check the line-of-sight a few times, then go back to sleep. With a switching (or no) regulator and some intelligent power budgeting in the design phase, you'll easily get this to run for days on AAs.

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Use a 555 timer to output 38khz to an infrared led and use an IR detector to output a high signal when the led is pointed at the detector. when the beam is broken, you can trigger an interrupt on the micro. There are lots of details on this ladyada tutorial

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