piezoelectric flapper manufactured by Measurement
Specialties, Inc. The LDT0-028K is labeled as a vibration
sensor or as a piezoelectric switch. It consists of
Polyvinylidene Fluoride (PVDF) — a piezoelectric plastic
with metallic electrodes. If its plastic tab is bent over 90
degrees, the LTD0-028K will generate up to 7 nC of
charge, or 90 volts. In Figure 3, we use a plastic
piezoelectric capacitor to switch a ceramic ferroelectric
capacitor in an event detector circuit.
There are issues using the LDT-028K bender as shown
in Figure 3. It is a capacitor, so when sharing charge with
the Type AD ferroelectric capacitor, it will generate the
voltage set by the ferroelectric capacitor for the amount of
charge being deposited. If the ferroelectric capacitor starts
DOWN, a bend of the plastic piezoelectric switch
generates 7 nC to force the capacitor UP. According to
Figure 2, the voltage will only reach about six volts.
However, if the ferroelectric capacitor is already UP
and does not switch, the switch could generate a far
higher voltage across the ferroelectric capacitor and
damage it. Even more important, if the plastic
piezoelectric switch is bent the wrong way, it will generate
Another issue with the simple circuit in Figure 3 is the
status of P0 and P1 when the microprocessor is powered
down. All pins on a microprocessor package will have
clamping diodes to the power rails (Vcc and ground) to
dissipate static discharge inadvertently applied to a pin.
When the LDT0-028K attempts to generate a
voltage on P0 to switch the ferroelectric
capacitor UP while the microprocessor is
powered off, the clamping diodes on P0 will limit
the voltage range to ±0.7 volts.
A blocking transistor must be placed on P0
to isolate the external components from the
clamping diodes of P0 inside the package when
the microprocessor is turned off. On the other
hand, we can use this property to our
advantage on P1. When the piezoelectric switch
attempts to change the state of the ferroelectric
capacitor and the microprocessor is off, the
ferroelectric charge will flow into CSense until the
voltage on CSense exceeds the clamping voltage on P1;
whereupon, the excess current will flow to ground. We
will not have to worry about back voltage from CSense
fighting the voltage across the ferroelectric capacitor
applied by the switch. To solve these issues, we add three
components to the event detector circuit in Figure 3 to
create a realistic real world circuit in Figure 4.
T0 isolates the common node A from P0 and is only
enabled by the microprocessor when it wants to write or
read CFE. When power is off to the microprocessor, T0 will
be off to allow the piezoelectric switch (LDT0-028K) to
generate a voltage across the ferroelectric capacitor and
not be clamped to ground by P0. When power is on,
event detection will still function.
T0 is turned off during detection. P0 and P1 are left at
zero volts. Sensing pulses from the LDT0-028K are
blocked from P0 by T0, but current from CFE can flow into
P1. D1 — at all times — blocks negative voltages from the
sensor, and the TVS limits the voltage across the
ferroelectric capacitor to no more than + 6 volts.
Recommended components for the detector are:
1. Radiant Technologies Type AD 10,000 µm2
2. 680 pF CSense
3. 2N7002 N-channel FET for T0
4. SMAJ- 6.0A for the TVS
5. 1N914 for D1
50 October 2015
FIGURE 5. An event detector circuit
on a perfboard.
FIGURE 4. An event detector circuit complete with overvoltage protection.
FIGURE 3. Simple event
detector circuit and the