Choosing a device for analog

[SPARKY49]

I am interested in making a gear indicator for a bicycle.
The rear derailleur is not visible.
Can the Raspberry pi measure the rate of the pedal and wheel rotation and calculate the chain positioning (assuming that there is no overlap in the ratios)?
It will need to be able to keep track of pulses from two inputs and do maths at the same time.
Thanks for reading.
Cheers!

[drgeoff]

Yes a RPI can do that. The hard part will be getting the suitable pulses into the RPi.

[boyoh]

You could use magnets and reed switches to switch pulses
to the Pi, Fit the magnets on the moving parts . and
Reed switches on fixed parts,
3.3v--------( RS )--------GPIO/IP--------( 10kr )------0v-
RS = Reed switch
10Kr = Pull down Resistor
3.3v Rail

[JohnsUPS]

Here are a couple of additional suggestions for obtaining pulses:

Affix a magnet or two to the rotating part (wheel and front sprocket), and locate hall effect sensors so that they switch when the magnet travels by it. Count the pulses of each, and do the math in software.

Affix an optical pickup so that the spokes of the wheel break the beam.

Neither would incur contact bounce.

[NGC6543]

Is the gear change cable open, and if so (or even if not) might it be simpler to have a linear position sensor on it?

Maybe something like a slider potentiometer with the slider pinned to the cable.

[Brandon92]

Is the gear change cable open, and if so (or even if not) might it be simpler to have a linear position sensor on it?

Maybe something like a slider potentiometer with the slider pinned to the cable.

I would also go for that option. Or inwpuld take a look at the part that you select the gear to see if you can a a potentiometer there. And I would use a microcontroller, arduino, in stead of your Rpi to detect this.

[PhatFil]

Is the gear change cable open, and if so (or even if not) might it be simpler to have a linear position sensor on it?

Maybe something like a slider potentiometer with the slider pinned to the cable.

I would also go for that option. Or inwpuld take a look at the part that you select the gear to see if you can a a potentiometer there. And I would use a microcontroller, arduino, in stead of your Rpi to detect this.

or attach a cable tie to the exposed cable with its tail extending over the top of the bar on which you have numeric stickers again it only works with exposed cables..

[DougieLawson]

You're probably best off by sensing the position of the two derailleurs relative to the frame of the bike. If you know the front chain ring teeth number and the rear sprocket teeth number it's easy to calculate the ratio (and a bunch of other numbers).

There's a web page doing that in Javascript at: http://machars.net/bikecalc.htm