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Here’s another Make: Books PDF excerpt. This one comes from Tom Igoe’s Making Things Talk, which tells you how to create Arduino- and XBee-powered devices that form networks of smart objects. This excerpt shows you how to work with two different distance rangefinders, the Sharp GPxx series of infrared rangers and the Devantech SRFxx series of ultrasonic rangers:

Electronic locating systems like GPS, mobile phone location, and sonar seem magical at first, because there’s no visible evidence as to how they work. When you break the job down into its components, it becomes relatively straightforward. Most physical location systems are based on the same principle. They determine distance from several known and fixed locations by measuring the energy of an electromagnetic or acoustic wave coming from the object to be located. Then they combine those measurements to determine a position in two or three dimensions.

For example, a GPS receiver determines its position on the surface of the planet by measuring the strength of received radio signals from several geosynchronous satellites. Similarly, mobile phone location systems measure the signal strength of the phone at several cell towers. Sonar and infrared ranging sensors work by sending out an acoustic signal (sonar) or an infrared signal (IR rangers) and measuring the strength of that signal when it’s reflected off the target.

Distance ranging techniques can be classified as active or passive. In active systems, the target has a radio, light, or acoustic source on it, and the receiver just listens for the signal from the target. In passive systems, the target doesn’t have to have any technology on board. The receiver emits a signal, and the signal bounces off the target. Mobile phone location is active, because it relies on the phone sending out a radio signal. Sonar and infrared ranging are passive, because the sensor has to emit a signal in order to measure the reflection. GPS is an active locating technology, because although the receiver doesn’t emit a signal, it has an electronic receiver onboard to receive satellites’ signals.

Sometimes distance ranging is used for acquiring a position, and other times it’s used for refining it. In the following examples, the passive distance rangers deliver a measurement of physical distance, but the radio ranging tell you only when another radio is in transmission range of your radio, and whether it’s near or far within the range…

Follow this link for the full excerpt.

Previously:

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Buy Making Things Talk by Tom Igoe in the Maker Shed today!

Programming microcontrollers used to require an expensive development environment costing thousands of dollars and requiring professional electrical engineering expertise. Open-source physical computing platforms with simple i/o boards and development environments have led to new options for hobbyists, hackers, and makers. This book contains a series of projects that teach you what you need to know to get your creations talking to each other, connecting to the web, and forming networks of smart devices.

Brian Jepson

I’m a tinkerer and finally reached the point where I fix more things than I break. When I’m not tinkering, I’m probably editing a book for Maker Media.


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Comments

  1. Andrew says:

    There’s a bit of an error in the way the first paragraph explains how GPS works.

    GPS doesn’t determine position by measuring signal strength and using rules about signal strength over distance. GPS calculates the distances to the satellites by using the travel time of the signal from satellite to receiver and the speed of light, and then triangulates in the _four_ dimensions of space-time.

    This requires insanely accurate clocks, compensations for inaccuracies they can’t control, corrections for relativistic effects from the speed the satellites are going (slows their clocks, an effect of special relativity) and their higher position in the gravity well of earth (speeds up their clocks, an effect of general relativity), and average ionospheric delay of the signal.

    Still the same base principle to determine position – get distances and triangulate – but it uses a different method to get the distances than is described.

  2. Brian Jepson says:

    Thanks, Andrew! I’ll submit this to the book’s errata page.

  3. Tien Gow says:

    “The receiver emits a signal, and the signal bounces off the target”

    No! Receivers receive. Transmitters emit signals. What Igoe is attempting to desdcribe is how radar works, but not GPS or cell phone location. These are passive devices that analyze signals emitted by other devices, namely satellites and cell phone towers.

    The book sounds interesting, but basic mistakes such as this and the one Andrew addressed above make me wonder how much Igoe really knows about his subject.

  4. Brian Jepson says:

    Tien Gow, thanks; You are right that receivers receive, but it would have been slightly confusing to refer to it as a transceiver, since the component he’s discussing is talking only to itself. Check out the PDF and you’ll see that he is segueing from GPS and cell location systems into the distance rangers used in the excerpt.

    This book is primarily about making things with Arduino and related technology. Tom’s a core member of the Arduino team, and if you check out the rest of the PDF as well as Tom’s body of work, you’ll see that he knows the subject inside and out.

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