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
Bicycle Anti-Theft Alarm Circuit Diagram
Bill of Material
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
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.
Single JFET transistor charge amplifier
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.
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