# Pedometer-like device

## Main Question or Discussion Point

Hello all,

I'm trying to set up a system to calculate how far I've run (more accurately than a pedometer, which just counts steps). I basically need a sensor in each shoe, and one of the sensors sends a signal that tells how far apart the two sensors are. Whenever the distance decreases, I take the previous (and largest) value and add it to the total distance. If the end result is off for whatever reason (like a weird gait, for example) I can divide the distance by a scalar.

I've been looking for small sensors that can detect the distance between themselves. If they exist (in a small size - hopefully the circuit board isn't more than 1 square inch/6.5 square centimeters), one would probably ping the other using some frequency and time how long it takes to get a response. I'm only trying to measure distances no greater than about a meter, so these sensors would have to be somewhat precise. And finally, as this is my first project of this sort, I'll probably be using an Arduino, unless somebody has a better suggestion.

Are these devices sold? Does anybody know what they'd be called? I could probably find where to get them if I knew the term to search for. :)

Thanks in advance for any help. :)

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NoTime
Homework Helper
Welcome to PF.

I don't know of any off the shelf sensors like this.

Perhaps you could use two accelerometer chips.

Then again why not just buy a GPS?

f95toli
Gold Member
Commercial "Foot pods" (as they are often called) use a variety of techniques to increase precision (including accelerometer) and you are right in that they are better than pedometers; but they are still nowhere near as exact as a GPS.
Nike claims that their Nike+ sensors are something like 95% accurate, but 5% is still quite a lot in running.
I suspect the main problem with a DIY solution is that it is likely to end up being quite big and heavy, the batteries might be another problem.

A small GPS would probably work. I have a larger GPS device, but I see they sell ones intended for runners, so it might be a practical alternative. I'll keep it in mind.

On a side note, the Nike+iPod is cheaper than I remembered it being. Of course, you need Nike+ shoes and an iPod Nano, and those are expensive (but not uncommon) prerequisites. According to HowStuffWorks, the Nike+iPod does use accelerometers, as you said, to approximate stride as well as knowing turnover (steps/time). No wonder they're inaccurate. Still, it's a good option, so thanks for the suggestion.

I'd still like to shoot for a transmitter system that measures the distance between the sensors. Here's what I have in mind:
• first device:
• a frequency A transmitter triggered by the Arduino
• a frequency B receiver that sends a signal to the Arduino
• second device:
• ...that triggers a frequency B transmitter
• Arduino loop (no time delay):
Code:
float dist = 0;
float a = 0;
time t = response.time - request.time;
float b = WAVE_SPEED * t / 2;
if (b < a) {
dist += a;
}
a = b;

The precision of the sensors would have to be tremendous if they were to use electromagnetic waves. At, say, a separation of 1 meter, we get
Code:
1 = [b][i]c[/i][/b] * t / 2
and t is about 7 nanoseconds. And neither Arduino nor Boarduino boards even have clock cycles below 50 nanoseconds, let alone the capability to time anything to the nanosecond. Even desktop CPUs have no more than a few clock cycles per nanosecond.

How possible/reliable would sound wave transmitters and receivers be? At least sounds travels at a sane speed, although that speed does vary, which could be a problem.

Having done the math, finally, I can see why nobody has built and sold foot pods that use radio sensors. Thanks for your help, as I think I'll have to investigate GPS or accelerometer options after all. (For the latter, I don't have the know-how to model stride based on information from the accelerometers, so I would probably have to buy a foot pod.)

Borek
Mentor
Look for GPS data loggers, something like Mainnav MG-910D, or MicroDot Squared, or Qstarz BT-Q1000P or Holux M-241.

I'm currently poking about for sound transmitters, but that term only turns up FM transmitters. Since I want to use sound waves, I'll probably just use a small speaker and sound sensor. According to http://en.wikipedia.org/wiki/Image:Landrange.jpg [Broken], no land mammal can hear sound over 110 kHz, so that's the pitch I'll use (if I can get the appropriate pieces), since I don't want dogs and rodents chasing me down the streets. Fortunately, high pitches tend to accompany small sizes, so it may be possible to generate that pitch. Getting the equipment may be a long shot, but accurate gauging of distances, the price (hopefully), and the do-it-yourself factor are appealing.

Thanks for the suggestion of GPS data loggers...I see they come cheap (around \$40 for bottom-end ones). But they're a little too practical for my taste.

On a side note, I haven't been able to find a wholesaler of electrical components - an equivalent of McMaster-Carr but not for hardware, if you will. Radio Shack and McMaster-Carr have a few parts, but are there any stores that sell everything?

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