Tilt/fall detector has staggered thresholds

Measurement-and-control applications may require 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 Figure 1 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.

The voltage divider comprising R_S, R_A, and R_B 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 α_TA=45°. The lower reference, with respect to the midvoltage supply minus V_REFB, corresponds to the upper-threshold tilt angle, where α_TB=60°. If you choose a value of 100kΩ for R_S, then you can calculate R_A+R_B:




The Z-axis voltage, V_GZ=300mV, occurs when the accelerometer’s Z axis is oriented vertically. From the obtained value of R_A+R_B, you can calculate R_B:




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

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 1pA. 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 IC_3A and IC_3B, respectively. NAND gate IC_3C serves as an inverter, and the output of IC_3D is the logic output of a window comparator in which logic low appears only when the Z-axis output voltage is between V_REFA and V_REFB, referenced to supply midvoltage V_S/2.

Grouping the comparators into IC_3A, IC_3B and IC_3C, IC_3D 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
1. “ADXL335 Small, Low Power, 3-Axis ±3 g Accelerometer,” Analog Devices, 2009.
2. “AD8603/AD8607/AD8609 Precision Micropower, Low Noise CMOS, Rail-to-Rail Input/Output Operational Amplifiers,” Analog Devices, 2003 to 2008.
3. “HLMP-EGxx, HLMP-EHxx, HLMP-ELxx New T-1¾ (5mm) Extra High Brightness AlInGaP LED Lamps,” Avago Technologies, AV02-1687EN, April 21, 2009.

Caption

Figure 1: An accelerometer’s Z-axis output, compared with two reference voltages, can generate two digital outputs.