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Tilt/fall detector has multi-level thresholds

12 Aug 2015  | Marian Stofka

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Measurement-and-control applications may call for action based on two distinct voltage levels. Crossing a threshold can produce a warning indication, whereas reaching a higher threshold may initiate emergency action, such as a system shutdown. In a fall-detector application, an apparent decrease in gravity below a lower threshold might be a controlled displacement, but a further decrease below a second threshold might indicate an uncontrolled fall.

The circuit in the figure uses a voltage divider to generate two reference voltages. Comparators and Schmitt-trigger-input NAND gates let you create two digital signals based on using reference voltages VREFA and VREFB. The sample circuit drives two LEDs, but you can use the digital signals to drive transistors or relays, as well.


Figure: An accelerometer's Z-axis output, compared with two reference voltages, can generate two digital outputs.


The voltage divider comprising RS, RA, and RB sets the voltages for comparing the Z-axis output of an Analog Devices ADXL335 accelerometer (reference 1). The higher reference voltage, VREFA, corresponds to the lower-threshold tilt angle, where αTA=45°. The lower reference, with respect to the midvoltage supply minus VREFB, corresponds to the upper-threshold tilt angle, where αTB=60°. If you choose a value of 100 kΩ for RS, then you can calculate RA+RB:



The Z-axis voltage, VGZ=300 mV, occurs when the accelerometer's Z axis is oriented vertically. From the obtained value of RA+RB, you can calculate RB:



Based on the chosen values of the tilt angles, RB=(RA+RB)/. You can then solve for RA from the known values of RA+RB as well as RB.

The AD8609 op amp's input-bias current causes errors, but these errors are negligible because the input-bias current at room temperature is just 1 pA. The AD8609's input offset voltage, which is typically 50µV, also causes errors, which are negligible as well (Reference 2). The signals at the outputs of comparators IC2A, IC2B and IC2C, IC2D are ORed in NAND gates IC3A and IC3B, respectively. NAND gate IC3C serves as an inverter, and the output of IC3D is the logic output of a window comparator in which logic low appears only when the Z-axis output voltage is between VREFA and VREFB, referenced to supply midvoltage VS/2.

Grouping the comparators into IC3A, IC3B and IC3C, IC3D pairs ensures independent detection on whether the Z axis is 0 or 180° in the vertical orientation. LED1 and LED2 illuminate successively upon slowly tilting the Z axis by 45 and 60° (Reference 3). Similar action occurs when you orient the Z axis steadily vertically while moving downward. LED1's brightness is turned on at an apparent decrease of gravity to g/ . LED1 dims, and LED2

simultaneously illuminates when the vertical acceleration is equal to or lower than g/2. The operation of the detector is ratiometric and is therefore virtually insensitive to supply-voltage variations.


References
"ADXL335 Small, Low Power, 3-Axis ±3 g Accelerometer," Analog ('ue'?) Devices, 2009.

"AD8603/AD8607/AD8609 Precision Micropower, Low Noise CMOS, Rail-to-Rail Input/Output Operational Amplifiers," Analog ('ue'?) Devices, 2003 to 2008.

"HLMP-EGxx, HLMP-EHxx, HLMP-ELxx New T-1¾ (5mm) Extra High Brightness AlInGaP LED Lamps," Avago Technologies, AV02-1687EN, April 21, 2009.


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 February 18, 2010 in EDN.com.




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