when i do a knock piezo read correctly a value, it's ok.
but i have a problem with piezo and servo. when the servo is locked, il pulses randomly short mouvment that generate signal in the piezo and make a knock.
also if i force manually a mouvement to the servo, that generate signal to piezo.
i do the same circuit in the kit book.
an idea of what hapens ?
A reply several years later.
I am experiencing the same. Movement of the servo sends a back current that registers as piezo signals. Jiggle the servo arm and that's enough to open the lock. Here is a video showing the issue...
Ideas for how to fix this?
I had the same problem on this project. servo is making interferences that are readed as knocks and open the lock.
Servo buzz can come from two thing:
peak impulse when starting the servo, solution is putting an electrolytic capacitor, at least 100 uF, directly across the power supply to the servo.
I tried it and it didn't change anything.
second solution: "cheap servos don't know how to stay at the limits of their movement very well, which can cause the buzzing/shaking you're mentioning. However, if you just limit their range to 10~170 degrees, the issue will be fixed."
I did it and it worked!
So the servo bundled in the arduino starting shouldn't be used at limit positions.
all credit to http://electronics.stackexchange.com/questions/77502/is-there-a-way-to-stop-servos-from-shaking where I found thoses answers