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Detecting X-axis position

11 Sep 2015  | Marian Stofka

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Gravity causes a voltage difference at the ZOUT pin of IC1. The circuit detects fall on the loss of this gravity-induced voltage difference within "free fall"—moving bodies with no acceleration other than that provided by gravity. If the circuit is fixed to such a body while the Z axis of IC1 is pointing roughly vertically, the free fall manifests itself as almost fully disappearing within the 300- or –300-mV voltage excursion at ZOUT. When the voltage remains close to the power supply's midvoltage, the voltage at ZOUT is 1.5V. The threshold of detecting the free fall in this case is an apparent decrease in gravity to 0.4545g.

The probability that the noise's peak value will achieve this threshold value is practically zero for "heavy" bodies. The probability that the noise's peak value will achieve 0.0679g is fairly low, and it decreases vastly when you elevate the decision level. An apparent decrease in gravity within the free fall causes a low-to-high transition at the output of either IC2A or IC2B, depending on whether the Z axis is close to parallel or antiparallel to vertical. The outputs of both IC2A and IC2B remain at a high state. Thus, in both orientations, the output of the NAND gate goes low, and the LED turns off.

The sensitivity of IC1 is essentially ratiometric. The resistive voltage divider RS/RR derives the positive and the negative reference voltages, which are inherently ratiometric. Thus, the detector's operation is virtually insensitive to power-supply variations. Note that the NAND gate has an internal Schmitt trigger at its inputs, and its logic output therefore fulfills industrial-grade requirements, including duration of the logic-state transitions of no more than 10 nsec regardless of the slope of the detected signal when crossing the threshold. If you need a complementary on/off indication, you can reconfigure the circuit by another position of the LED (dashed lines in the figure).

1. "ADXL335: Small, Low Power, 3-Axis ±3 g Accelerometer," Analog Devices, 2009.
2. "Precision Micropower, Low Noise CMOS, Rail-to-Rail Input/Output Operational Amplifiers, AD8603/AD8607/AD8609," Analog Devices, 2003 to 2008.
3. "HLMP-EGxx, HLMP-EHxx, HLMP-ELxx New T-1¾ (5mm) Extra High Brightness AlInGaP LED Lamps," Avago Technologies.

About the author
Marián Štofka is with Slovak University of Technology in Bratislava, Slovakia.

This article is a Design Idea selected for re-publication by the editors. It was first published on December 3, 2009 in

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