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How would I make a cheap displacement sensor?

I was thinking about putting a plunger against a slide pot with a spring to reset the pot, or making a plunger that rolls a rotary pot with a spring, but I was wondering if there was a way that I could use a piezo like this

http://www.ebay.com/itm/10-24mm-Cased-Piezo-Disc-Projects-Unltd-Audio-Transducer-3-Lead-Buzzer-Alarm-/120900133885?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D120961575052%26ps%3D54

I think I could have a plunger push down on a spring that touches the exposed part. I don't need to do anything with the signal other than read the voltage and even then I just need to know what it's at compared to the range of values from pressed down to not.

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iceng3 years ago
Lets first eliminate the piezo disc ... which mainly measures pressure not displacement and
tiny flexes as a voltage output.

Unless your displacement compresses a spring that increases pressure and thereby
piezo output voltage in proportion to said displacement  and you have a load cell :-)
tjmburns (author) 3 years ago
Okay, I was an idiot and just remembered that a piezo only outputs when it's flexed, not when it's holding at a certain amount of compression right? I think I'll just have to go with one of the pots. I'm worried about how close I can get them together. This is for a musical instrument with a harmonic table layout similar to the axis49, but I want the keys to read how far they are pressed in to control the volume of each key. Load cells and pressure gauges are apparently way out of my price range for this project as I need to make 98 keys.

Anyway, thank you for responding :)
iceng tjmburns3 years ago
Actually deformation of a piezo polycrystalline structure displaces charge ie voltage.
Easily measurable your spring force concept should work !
Yep, and that charge has a despicable habit of leaking away :-(
what are you building see to really answer a question we need to know the work being done such as filling a tank with water and shutting off the tap when full?