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Practical circuit to detect contact with vibration/variation in tone amplitude?

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I have an idea I want to try for 3D printer bed leveling and Z-homing.

A usual contact probe requires a way to automatically move it out of the way of the print without messing up the precision, it also requires a precise offset value for the printer to use for correction. So this makes it attractive to use the printer nozzle itself as the contact point, but generally mounting load cells in the bed or the end effector mounting is required, although this is probably the best method for now. But suffice it to say that even if this isn't a better method I'm suggesting, I'd still like to try it and see how well it works.

What I want to do is put a vibration inducer, ultrasonic or otherwise, on the print bed, and put a pickup on the print head. Then the print head will move down slowly when homing and stop when the pickup detects increased vibration. I say increased because it's bound to pick up some through the frame of the machine.

I know that for many types of signal processing the task of detecting a known tone and measuring it's amplitude is required, or at least detecting rising above a certain threshold. For example in any amplitude shift keying protocol.

So I'm asking about approaches because I know there are several ways of doing this. I can think of a few that could probably work. Since I work with micro controllers the most obvious thing is to use one for signal processing, but without a filter circuit of some kind this would take a very high analog sampling rate, and I'm sure there is some discrete circuit that is well tested for this exact application.

Along with this, what would make the best pickup and transducer? When I first pictured it I was thinking of a speaker coil and microphone but I suppose there are lots of ways, a piezo transducer or pickup. I've been looking at circuits used for ultrasonic pickups and ASK receivers but it seems like some outside input could really help on this idea.

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