on a door in the castle
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I have similar plans. I have a handful of piezos , rando bits of wire, and 1/4 jacks. Time to find some fun things to jack in.
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Like an old fire alarm bell?
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Just noting a thought I had re the door stopper. Wondered if the piezo element could be replaced with an IR transmitter / receiver pair and an opamp. Using the doorstop or string, spinning disk, jelly … anything really to interfere with the carrier and produce an analogue signal. Up the gain for a square wave or gate.
I sketched a plan for a hexaphonic optical guitar pickup, using the same idea, as an audio and MIDI interface. (It’s on the "when I’ve got better at 3d printing from my own sketches " pile) Never precious about these things so if anyone has the time to try it? “Gaun yersel!”
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Or something like this ?
Without the sound making chip, and OpAmps for the LDRs…
I have a similar kit waiting to be built in one of my “todo” boxes.
It looks like this, was made by sainsmart IIRC.
Could replace the strings on a guitar 
EDIT:
Looks like they are still available :
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Similar in parts. I’ve built that first kit for a Christmas present. However these are trigger devices. Im suggesting that the stopper or string’s vibration casts an oscillating “shadow” on the receiver.
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Interposing semi-transparent or moving things in the laser beam gets you an analog signal on the LDRs, exactly as with IR.
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The kit I built was certainly just an on/off but yes the laser would work as well though I thought of using a phototransistor rather than an ldr.
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Yes, the sainsmart also is just on/off.
But you should just get rid of the sound making chip and adding OpAmps to buffer/scale/etc the LDR signals.
Phototransistor would be OK too I guess.
But laser+LDR will make a… lasertrol
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Okay this is my last video about the doorstopper. It’s weird. I don’t even know what to say about it.
I was trying out a new video editor as well (so video/audio are actually in sync). Too bad my camera work was pretty awful. 
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very cool !! 
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I hope it isn’t your last video featuring the doorstopper. You’ve created a really great sound there in that jam.
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Related:
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nice and yes works very well even with real large spring reverb tank .
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