Wiring And Schematic

I hate to suggest the specific application ‘bicycle’ because it may be use to protect many items from theft. This anti-theft alarm project is built around the inexpensive Measurement Specialties DT piezo film sensor. Every now and then everything seems to work out perfectly as in the Yin and Yang of the cosmos, and this is one of them. It is simple, inexpensive and practical

 Piezo sensor

I received this DT piezo film sensor as a sample years ago. It was attached to the application page via a round sticker. I never removed the sticker, but used it to attach #6 nut to increase inertia at the tip of the device—I could have experimented with other small masses, but this worked well from the git-go, so I left it that way. When the film is flexed, it produces a voltage at the terminals.
http://www.meas-spec.com/downloads/DT_Series.pdf

Initial experiments with the sensor were disappointing—I observed voltage and connected it to a charge pump type detector—yes, it functioned, but sensitivity was poor. 

Then I read up on charge amplifiers. One good discussion is “Signal Conditioning Piezoelectric Sensors” http://www.ti.com/lit/an/sloa033a/sloa033a.pdf

The paper discussed using FET input op amps in such a way that the sensor develops no voltage at the output terminals—only generates a current that is then amplified by the charge amplifier. This is important because the sensor has significant capacitance (e.g. 500pf) and any voltage generated by the sensor is swamped by this capacitance thus greatly attenuating the output voltage.

Then the wheels started turning—and I thought up a means of using a single JFET as a charge amplifier. This I bread boarded and tested—performance was phenomenal! The source feedback resistor doubles as a negative feedback device depending upon the position of the trim pot adjustment.