All,
I have been wondering about this - what could I use to pick up the vibrations of a tap on some glass. A piezo?
Imagine the piezo sensor on one side of the glass - and someone tapping with a finger on the other. Is this detectable?
James
Some accelerometers have tap detection built-in.
A piezo might be a little difficult. Mine tends to have a low analog signal (maybe 50?) when you knock it hard with your knuckles. While that isn't a problem, as you could modify the threshold in your code, it might pick up unwanted noise. How thick is the glass? Are we talking like a window? I think if you secured the piezo very tightly to the glass it might work. Guess and check, I suppose.
Jeth
While you might not get enough signal from a piezo you could try an ordinary microphone, mind you you would probably need an amplifier.
I have done this with reflected lasers off the inside of the glass. (Larger windows panes work better)
Mount a laser on the inside somewherre and aim it at the window, reflecting it to a photocell in a tube or behind a plate with a small hole on the other side of the window frame. You can also use a mirror on the other side to bounce the laser back to the sending side to extend the beam distance, therefore sensitivity and to keep all the components on one side of the glass.
When the knock is detected, it moves the glass slightly, causing the laser reflection angle to change, causing more or less light to the photocell. The larger the window and reflection angle, the more sensitive the detector.
This might be WAYYYY wayyy pie in the sky dreaming... but after posting this I had an idea...
Imagine 4 "tap" sensors in each corner of the glass (we are talking big here) - and then timing each tap with each sensor...
Now I imagine we might be talking about the speed of sound here... or more... speed of vibrations through glass...
Could you detect where it was being touched!?
Sorry - mad tangent! Probably impossible - but was a thought I had (this was to do with interacting with shop windows etc)
Could you detect where it was being touched!?
Yes, this is more-or-less how some early touchscreens worked.
Not easy though,
Maybe you could try the devices created by this company:
http://www.covialdevice.se/en/?PHPSESSID=618e45c3867a2516ce1ba4c1927d13cb
The Glassalert is probably something like what you are looking for (tapsensing)
/Kristian
I believe the "tap" sensors they use for burglar alarm systems are essentially piezo microphones.
I'd start with shopping for those: there are probably some that already have the conditioning circuitry needed to turn the tap into a digital pulse (though maybe at the 12V commonly used in alarm systems).
ran