Aim:

Making a cheap piezo disk work as sensor.

Need:

When it came to building the sensors for another project (https://hackaday.io/project/3336-wifi-sensors) I could not find any that made sense financially... The entire sensor device including the housing was costing less than $10 but the sensor was costing $20 to $30 or more.

Prior Knowledge:

First it must be understood that vibration sensor detect movements as a result of a force applied, They do this by restricting the movement of one side of the sensor and allow the other to move freely... It's the movement of one side in relation to the other that produces the output signal...

Reasoning:

In previous designs I experimented with inexpensive piezo disk but as these are primarily designed as output devices their use as sensors (vibration/movement) is somewhat lacking. The problem as identified above is that both plates of the piezo disk are effectively fixed and one can't really move in respect to the other...

A piezo disk would certainly detect vibrations from a window breaking but almost certainly not the small vibration resulting from opening a window or cutting glass.

Solution:

If we attach a small weight to one of the plates by means of a rigid wire, any vibration experienced by the "sensor enclosure" will be transmitted to the sensor causing it to vibrate. Within a very short period all vibrations will dampen leaving the weight (which is not dampened to the same degree) to continue to vibrate causing the plate to vibrate and generate an electrical signal...

(Mechanical amplification or vibration)...

Not sure if I explained it well but it might be worth pursuing especially if you require many sensors and don't want to fork out hundreds.