project 12 - knock lock: problem piezo read pulse of the servo

Hi everybody,

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?

Hi. 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 where I found thoses answers