We'll start with the little alarm.  Take it out of the package (might want to save the batteries, if you have anything like a laser pointer or something they'd fit) and take it apart.  Sometimes there's a little screw, but this particular type is just snapped together and you can carefully pry the back off it with a screwdriver.  It will look something like this:

If you lift up the circuit board, you'll see a sort of metal disk thing.  Cut the wires on it off, leaving them as long as you can.

Ok, now it is time to get destructive!  LOL  Use a pair of pliers and carefully break away the plastic until the metal disk comes loose.  It isn't *super* fragile, but try to avoid bending it too much or denting the crap out of it.  When you have that out, you can carefully scrape off any excess glue around the outer edge with a knife.

That, ladies and gentlemen, is the piezo element.  All we need to do now is provide a way to hook it up to your computer (or other sound device like a guitar amp or whatever).

By the way, you may have seen something that looks just like that in some other place, like a musical greeting card or a small toy that has sound effects built into it.  Those usually will work too, and if you happen to have one lying around, it can bring the price of this project down to even less than 2$!

Couldn't find any poster putty, so I ended up just sticking it on with a little piece of beewax.

I didn't play around with placement, just sort of stuck it on somewhere near where I've seen piezos attached to acoustic in some pics.

Well, it makes a sound..

Not the sort of sound I'd usually like, but moving it to different spot on the violin, even moving it less than an inch could fix that.  Every spot on the top of an acoustic violin vibrates favoring different frequency ranges, and some experimentation would probably find a sound I find more pleasant.

I already have an electric, and don't have much interest myself in "electrifying" my acoustic, so I'm not likely to mess with it much. 

But if one didn't have an electric or a pickup, maybe not a good microphone or perhaps didn't have a microphone at all?  This could be a very workable starting place.  A bit of playing around with trying different spots on the violin to find the sound one likes best, and then either the EQ on  a guitar amp or PA system or in recording software.. Some noise reduction to take out the hum, maybe some reverb or other effects, and it could get a very usable sound for at least beginning with some recording or making a track to get in on the group projects here on Fiddlerman.com

For all that it was cheap and home-made, it works.  With some messing around, it would work better. 

With piezos in general, most of them will sound much better with a pre-amp and some tone shaping circuitry. 

But if you don't have that, using the noise reduction and EQ in some recording software like Audacity would allow a crude little pickup like this to at least end up providing a usable recording. 

For running into an amp or through effects for a live sound, I'd want at least a simple pre-amp circuit on it to get a better match of the impedance and level for an amplifier's inputs.

On the other hand, if this little 2$ rig was all one had, it can at least be workable for recording a violin with.  Might actually get a better sound using the microphone on the earpiece, as per the other thread.  But this is a cheap entry to being able to try a piezo.

It is not all that silly an idea.  Some "stereo" pickups use a transducer under each bridge foot.  The sound is going to be a bit different under each foot and by directing them to different channels (basically a separate amp or amp channel and speaker for each).

There been guitar pickups designed with 6 or more different coils to pick up each separate string and run them into a mixer so they can each be eq-ed and a mix can be made of them.  So the principle has been thought of and tried at least somewhat.

But none of those sorts of ideas ever really took off huge with enough players to get enough market to become a standard option.  Multiple amps or even a stereo amp for performance amounts to more expense, more time to set up, and more that can go wrong.  Also a mountain of gear can distract from the performer or be a pain to actually run at a live performance. 

It is not a silly idea though, RosinedUp.  The only drawback I can think of immediately to putting 4 transducers on the bridge would be that the added mass could change and damp the sound of the instrument somewhat.  But very small ones, and some clever mounting, and I'd bet it could be done and still sound good.

Nothing at all wrong with that, either, Ferret.

While a few brave soul may try the route of cobbling their own together, I think that most people would prefer a nice pre-made unit.  This was more a demonstration of what can be done for a couple of bucks, with no real electronics or soldering. 

Honestly, I don't feel the sound quality of the 2$ piezo explained here is what I would want for recording.  But it would be better than nothing for someone who couldn't afford more and it might actually have some neat sounds through guitar effects boxes, if someone had just been dying to try that.  But even the primitive form explained here could work for recording, with a bit of noise reduction and tweaking the tone with the equalizer available in Audacity.

Myself, I would use maybe 5 more parts and solder together a particular type of preamp to go with the homebrew piezo, and that can actually sound pretty good.  But not everyone has soldering gear or experience with building even simple circuits. 

For someone just looking to record, the 1$ condenser mic in the other thread sounds better to me for contenders in the "extreme budget" range and is less work/cobbling.  And for someone who definitely wants a piezo pickup with much less fuss, Fiddlerman has some nice options available over in the Fiddleshop. 

I prefer a simple FET preamp, since op-amps can have their own problems, and I prefer how FETs react if they get overdriven a little by the input.  FETs also are easier to get to work well with very high impedance sources like piezo elements.  At least in my experience.

Op-amp chip or simple bipolar transistor circuits are apparently common in at least a lot of electric violins, though.

A preamp can be very cheap to build, if you are used to cobbling together a bit of circuitry.  Otherwise, there are some available for sale at assorted places on the internet.

Now, a tube/valve electric violin could be a dang fun concept, though perhaps a bit "steampunk".