A patented design for an earthquake detector is set to become a commercial product. Zoltech is building the instrument, called a volksmeter, that it hopes to bring to market by early in 2006 and is expected to become the least expensive earthquake detector in the world with a price of under $500 for a basic version. Fully functional seismic instruments of equal sensitivity are priced in the $10,000 range. The design uses a unique means of varying the surface area of a capacitor. The capacitors gap is constant. Therefore, detection is not accompanied by a drop-off in sensitivity, as happens with other capacitive sensors. When the gap widens, most of those sensors lose sensitivity. Several variations of a type of symmetric differential capacitive sensor have been patented and can be thought of as fully differential. In traditional capacitive sensors, dynamic range and sensitivity must be treated as a design trade-off. In this design, sensitivity is set by the constant size of the gap. Changing the surface area causes it to separately determine dynamic range. To make the design such that anyone can use it, Zoltech has used autozeroing electronics, which allows the device to work without calibration even if it is not completely level. When all four capacitors are zeroed, they start with the same surface area, giving a zero-rectified dc output from the bridge. However, movements of only 25 nanometers are sensed by the differential op amp.