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I'd like to build a speed detector that can measure speed of a running athlete, or the speed of serve in tennis or any other similar amateur sport. Devices like these are used during Olympics, Wimbledon, etc. Yes - it's all in spirit of inspiring a generation! :-)

I've seen Arduino based solution that relies on tripping sequence of light beams to calculate average speed - however that is too restrictive and will only work for events like running but not tennis. Hence I presume I need something like a radar sensor. Do these exist in reasonable hobbyist package? How do I get one? What're the best ways of hooking it up to the RPi?

Additionally, are there any alternatives? Would ultrasonic have a sufficient range? Will the doppler effect with ultrasonic reflection be easy enough to measure? Any other choices?

N.B. Sorry, but answers with light-beam tripping won't be accepted. :-)

  • WHy not just buy a speed radar or like this. Either way to make a nice non light tripping one it will cost a bit. – Piotr Kula Aug 13 '12 at 9:34
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    Looking for something in more amateur price range - something cheap enough to be used at sports carnivals. I've now found some Radar speed sensors at conrad-uk.com/ce/en/overview/0231510/Radar-sensors. However still not sure about their range. Anyone got experience with these or others? – Maria Zverina Aug 13 '12 at 9:52
  • Nice- a bit pricey but ok.. It says the range is up to 15metres but the spec sheetsays not to use where florescent lights are used. so maybe a laser / infra-red sensor is better – Piotr Kula Aug 13 '12 at 10:03
  • I think this is a bit off topic. You've mentioned the RPi at least, but if light-beams are too cumbersome I fail to see why you'd use the RPi over a notebook or tablet. You'll find a lot more expertise on radar etc. on electronics.stackexchange.com – John La Rooy Aug 13 '12 at 10:09
  • @gnibbler Trying to see if this is something that can be built together cheaply and without too much custom electronics. Hence basing it around RPi. As soon as you start going down laptop/tablet requirement, you might as well use custom made product as suggest by ppumpkin – Maria Zverina Aug 13 '12 at 10:35
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Hot Wheels Radar Gun

Some people have had some success hacking the Hot Wheels Radar Gun. The hack seems pretty in depth and a reasonable amount of analog electronics.

The guide tells you how to amplify the signal, but you have to develop a more complete solution yourself. For example, the frequency needs to be counted. This could be done in software on the Raspberry Pi or as additional hardware. Since the Raspberry Pi is running Linux, it would be advisable to do this in hardware and talk to the Raspberry Pi over an alternative interface, such as I2C/SPI. Such a digital output could be used by the Raspberry Pi or an alternative MCU.

The output from the hack uses 10Ghz signal using "Doppler Radar" technique; coincidentally, the same thing you have to do with the piece of hardware that you put in your comments[PDF], but that one uses 24Ghz. From the PDF

[At 24Ghz,] therefore, the frequency of signal originating at the mixer output is proportional to the speed: 44 Hz corresponding to a movement speed of approx. 1 km/h.

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By using either the suggested hardware in your comments (CON-RAD-MOD) or the toy radar hack you would have to ask your self why you want to use the Raspberry Pi?

An Arduino is perfectly capable, as is the Pi to decode the signals. As your question lacks a definitive question I will give an answer how to connect the devices to the Raspberry PI's GPIO.

The CONRADMOD output uses an open collector so you basically have to measure the frequency. Because the spec sheet already says it has a pull up resister your as safe to connect the output from the conradmod to a digital in pin on the Pi. You would have to read the toy hack guide in more detail how to connect it to a digital pin in a safe way.

So you would select a free GPIO pin, set it as input and read in a loop- calculating the frequency. The CONRADMOD outputs 44hz for every ~1km/h

But you may come to realise you need an external circuit for timing conversions as discussed in this post.

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For the tennis serve, what about a mic located at the net that hears the ball hitting the racquet, then the court. The distance of good serves is pretty fixed in terms of distance so you have that number. Two points in time and a distance can give clock speeds on tennis serves, right? I can't yet program the Pi but I wanted to share this idea anyway.

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Ms Kinetic would do it. But that's like light beam tripping.

  • How could I use that with my Pi? Could you expand your answer please. – Piotr Kula Aug 14 '12 at 12:23
  • Well, I guess it would also need a windows box to deal with the connect data but that cold be sent on to the RPi like the Arduino hacks do. vimeo.com/31698679 instructables.com/id/… – Hellonearthis Aug 14 '12 at 17:01

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