More Ferroelectric Magic
Ferroelectric capacitors make it possible to
create an electronic version of the hair-based
non-volatile event detector. Unlike more delicate
forms of non-volatile memory that depend on
static charges buried deep inside silicon dioxide,
ferroelectric capacitors can operate in the natural
world without being destroyed by Mother
Nature’s penchant for lightening large and small.
The February 2015 issue of Nuts & Volts
featured an extensive article describing the
physics of ferroelectric capacitors used as
memory elements. Demonstrated in the article
was a method of building and operating a simple
non-volatile memory using a ferroelectric
capacitor, a microprocessor, and a linear sense
capacitor. In this article, we will show how to
add a few more external components to that
same memory bit to allow it to detect events and
report them later.
Review of
Ferroelectric Memory
Following is a quick review of ferroelectricity.
For a more complete description, please re-read
the original February 2015 article (available at
the article link). The crystal lattice of a
ferroelectric capacitor is naturally polarized in the
same way that a permanent magnet is
permanently magnetized. This internal electric
polarization cannot be detected with a voltmeter
because the capacitor plates will always collect
enough of the opposite charges to exactly cancel
the polarization.
The internal polarization can be forced to
change its direction by moving the cancellation
We have all seen it so many times. Our
hero returns to his (or her) room to find a
hair fallen from the door sill. Out comes
the Walther PPK! That hair, placed on the
door sill before breakfast at a sunny
sidewalk café, saves our hero. It
functioned as a physical memory of the
status of the door. The hair detected the
occurrence or non-occurrence of a
specific event and stored that information
for retrieval by the spy returning to the
room. This hair-based non-volatile
memory had only two possible states
that could be instantly read by the
controller (our hero): in position (no
event occurred) or not in position (the
event occurred). Programming the hair
meant placing it at a specific position on
the door sill to be held in place by the
door. If the door was opened during the
retention period (breakfast), gravity let
the hair fall from its pre-set position. No
battery was needed for this event
detector!
Silent
Sensors
By Joe T. Evans, Jr. and Spencer T. Smith
48 October 2015
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