I need to detect when an irrigation head is functioning, non-invasive. The heads are the Toro brand and when activated a cylindrical shaft extends several inches above ground. This shaft is a non-ferrous metal, I assume to be polished aluminum. It measures 1.75" in diameter. My thoughts are to wind a coil about 2" in diameter of say, 50 turns of # 22 awg. This coil will sit slightly above and centered with the head. I will energize the coil with a square wave derived from a pic chip at say 1khz. Will I be able to detect a change in the applied waveform when the head's shaft enters the coil? Does the core (head shaft) need to be of a ferrous material? Is there a formula I can use to calculate the coil parameters?
The shaft would have to be ferrous to disturb the magnetic field of the coil so what your proposing wouldn't work with aluminum.
If all you need is water flow: Its possible that with a coil around the feed shaft or any other component which water flows through that you could detect a disturbance in the coil as a result of the moving water (which contains iron among other impurities in most locales). Your sensing circuit would have to be quite sensitive.
All that being said, if I were you I would just put a single flow meter on the system back at the water source and compare the amount of water flowing to the amount that would be flowing if all sprinklers were operating correctly.