Nuts and Volts - October 2015

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!