Turn FR into bike meter

Al Johnson openmoko at mazikeen.demon.co.uk
Thu Jul 2 20:32:01 CEST 2009

On Thursday 02 July 2009, Risto H. Kurppa wrote:
> Hi!
> I bike and I have a Freerunner.
> GPS apps, like omgps are able to show & record the position, speed,
> length of track etc.
> 1) I'm missing the cadence display = the number of revolutions of the
> crank per minute; roughly speaking, this is the speed at which a
> cyclist is pedalling/turning the pedals.
> -> any ideas how to build this, both hardware and software (=how to
> convince some GPS software guy to implement it :)
> I was thinking of using the audio jack or to be more specific, the
> microphone line. Each revolution would connect the wires (with a
> resistor maybe?) to create a pulse in the microphone line. This could
> be recorded and analyzed with SW.
> 2) Another thing missing is a tool to analyze my GPS track on-the-fly:
> show the speed vs. time plot.
> Thanks for your ideas! Extra thanks if I'm able to have this up and
> running in a week, planning to do a 170-200km bike trip in a day :)
> (more over, I think there could be a need for proper bike-oriented app
> for Freerunner. Map display's nice but many other features might come
> more important for people riding bike, like heart frequency, cadence,
> current speed, avg, maybe also set target length and estimate the time
> to reach it (I know Navit ~does this) etc.. -  anyone interested?!)

You could get a USB ANT receiver for ~$35 and write some code. It uses the 
FTDI usb-serial driver. There's example VB code for talking to Garmin 
heartrate monitors, and Garmin also have bike wheel and cadence sensors which 
you should be able to find on amazon if you search for "forerunner cadence" or 
similar. Suunto, VDO and others are doing bike and heart rate sensors using 
ANT too, and a few companies are also making power meters. The forum thread 
below contains linux c code for reading from Garmin, Suunto and other 
heartrate monitor belts. It's a bit old so there may have been some progress 
since then.


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