The SPUD has left the building

[Newz2000]

Can anyone suggest something I can google for or read that will explain how to do this? <p>I want a simple circuit that detects when the device is within about 20 feet of a specific sensor. The converse is also ok; knowing when the device is not within 20 feet of the specific sensor.<p>For an illustration, imagine that a mother places a sensor on her child (as a bracelet, for example). We'll call this sensor "the SPUD." In her purse is a small electronic device that sits around happily doing nothing unless the SPUD moves an unacceptable distance away (we'll say 20 feet).<p>The goal is ultra-cheap, and ideally, low part count. The size of the spud is not that big of an issue up to about the size of a baking potato. <p>It is also not terribly important that the spud be uniquely identifiable. So in the example above, as long as any child with a spud is within about 20 feet, that's OK. However, uniquely identifiable is a nice bonus. (yes, the example is contrived)<p>[ September 15, 2005: Message edited by: Matt Nuzum ]</p>

[JPKNHTP]

-JPKNHTP

[Newz2000]

Actually, I just realized this thread, "RF Signal strength to measure distance" talks about this very subject.<p>I was searching for "Homing device" and getting stuff way too high tech for this.

[L. Daniel Rosa]

A few years ago I saw something like this to keep your baggage safe from theft at the airport.

[Externet]

Hi Matt.
I have one pair for sale... never used; it's ~1" x 2".
Miguel
[externet at inorbit dot com]

[Newz2000]

Actually, I don't want to buy anything allready made. I'm going to investigate AGC this evening to learn more about it. From the older conversation it seems that's what's needed.<p>I just have to figure out what the cheapest/lowest part count technique for creating a brodcaster/receiver is.<p>The project is actually inspired by the fact that I keep forgetting to shut the garage door when I pull out of the driveway. But of course, after coming up with the bright idea of a beeper that goes off when you get a little ways away from the garage door, I started getting grandiose plans about how cool it would be if it could do X and Y and then, 2Y^3 (or f(x) as X approaches pi^3).<p>Please offer some ideas if you have some, but let's avoid redirecting this thread to fossil fuels, how mass production will ruin the environment in 3rd world countries or how big business sticks it to the little guys with superflous patents and monopolies. And please indent your code.

[terri]

Rope.

[Robert Reed]

"I just have to figure out what the cheapest/lowest part count technique for creating a brodcaster/receiver is."<p>You Might try a xtal radio set for your receiver and then expand as neccesary on that.

[Newz2000]

That's what I'm looking at now, but the world of rf strikes me as extremely complex, even just to dip your toe in the water.<p>This site seems to have schematics for simple trasmitters and receivers, but it's still way way way more than I was hoping for.<p>I'm thinking of something similar to those sensors on the backs of CDs that keep you from accidently walking out the door before paying.

[rshayes]

Radiating RF gets you involved with the FCC in one way or another. There are bands that can be used without licenses, but they also have limits on radiation that have to be complied with.<p>An alternative would be an inductive system operating somewhere in the audio range. A loop can be formed by running a few turns of wire around the door frame. A reasonable fraction of the magnetic field will still be found within a distance on the order of the dimension of the loop. It will drop off rapidly after that. If the loop is 8 feet high then a 20 foot range would be reasonable. A sine wave in the 10 KHz range might work without too much power required.<p>The receiving coil could be made smaller by using a ferrite or iron core. Tuning the coil to resonance will help. It could be mounted on the radiator grill as far away from the metal car body as possible. Since the signal is in the audio range, it can be easily amplified before being rectified. Diode rectifiers are not very sensitive at low signal levels.

[Gorgon]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Matt Nuzum:
I'm thinking of something similar to those sensors on the backs of CDs that keep you from accidently walking out the door before paying.<hr></blockquote><p>Hi Matt,
The sensors are not in the CD, but in the portal/ antenna at the entrance. The gizmo in the CD disturbs the RF field in the portal and sets off the alarm. The portal sounds off, both for entering and leaving the facility. <p>You could build something around the new Zigbee standard. The MCUs with the inbuilt hardware is not very expensive. I know Freescale have a variant. I've got a free demokit stashed away somewhere. Haven't had time to play with it. yet!!<p>Here you have intelligence in both ends, with addressable units. What more can you wish. <p>TOK <p>[ September 16, 2005: Message edited by: Gorgon ]</p>