While listening to embedded, a podcast on embedded development, the interviewee described how he was capable of sensing the pulses emitted from the common water meter. He did a project for the recent hackaday space prize, and got to the top 50 contestants.
The water meter uses a magnetic coupling between the flow blades and the geared counter within the meter. To avoid making any changes to the meter, he had to find a way to measure the magnetic field. This is where the idea becomes interesting, using a magnetometer . Since a magnetometer is capable of sensing magnetic north, it should be capable of sensing other magnetic fields. Sitting the magnetometer near the housing picks up the spinning magnetic field and translates in to a pulse. Although he said he had to hack it a bit to get the results and that a different setup would give better results, the proof was there.
This have me an idea for a measurement device. While researching optical encoders to find a way to measure small distances accurately, I was disheartened to see that the smallest collection slit available is .5mm and costs more than it is worth. I didn't spend too much time looking, so they could produce smaller ones. But I'm looking for a micrometer level measurement device.
I don't have time to do this now, but i will eventually get to this one. Attach a magnet or magnets to a shaft, spaced equally, and then use gearing to spin the shaft several times per mm. Then use the magnetometer to sense the field. I'm not sure, but if it is sensitive enough, a on off touch doesn't need to be the measurement. Potentially, if the magnets are calibrated to the same magnetic output, you may be able to achieve smaller increments by sensing the edges as well as the apex of the field.
Add this to the end of a digital caliper to roll along distances and then output as a keyboard. Possibly adding multiple buttons to label measurements and submit the text. Or use it as an accumulative measurement device. Roll, press save, roll press save, roll press save and send as output.
I had to put this thought out there, as a reminder to myself on the future.