Precision tilt/fall detector consumes less than 1.5mW

When you need to detect angular position related to Earth’s gravity, you can use an Analog Devices three-axis MEMS (microelectromechanical-system) accelerometer. The ADXL335 has three analog outputs that correspond to the X, Y, and Z axes of an orthogonal coordinate system (Reference 1). Because the Z axis is perpendicular to the footprint, or base, of the MEMS IC’s package surface, you can use the accelerometer to detect tilt if you mount it on a PCB (printed-circuit board) that’s parallel to your product’s base. The circuit in Figure 1 lets you detect whether the tilt exceeds a preset limit. A digital output, in this example, drives an LED, but you can connect the signal to a microcontroller or another device.


Figure 1

When you orient the accelerometer IC horizontally relative to Earth, the LED is on. Whenever the Z axis of the device declines by a specific value, αT,of the angle, α, from the vertical direction, the LED turns off. The voltage difference at the Z-axis output, ZOUT, of the accelerometer, referenced to the power supply’s midvoltage, VS/2, is VGZ=VGcosα, where VS is the power-supply voltage, VGZ is the voltage at the ZOUT pin, and VG is the terrestrial full-scale voltage. When the power-supply voltage is 3V, the terrestrial full-scale voltage is either 300 or –300 mV, depending on whether you orient the detector from the top down or from the bottom up. Op amp IC2 compares the voltage at the ZOUT pin to the reference voltage, VREF. If the positive voltage at the ZOUT pin is equal to or lower than the reference voltage, the output of IC2A goes high, and the output of IC2B remains high (Reference 2). Thus, the output of NAND gate IC3 becomes low, and the LED turns off. You can calculate the threshold tilt angle, αT, at which this action occurs from the equation cosαT=(VREF/VG).

Resistors RS and RR set the voltage reference to 136.36 mV. Thus, the threshold tilt angle is 62.96°. Similarly, when the negative voltage at the ZOUT pin becomes lower in magnitude than the negative reference voltage, it indicates a tilt of 62.96° or more, the output of IC2B goes high, and the LED (Reference 3) also turns off. Theoretically, you can choose any other threshold angle within the interval of 0 to 90°. The practical limits with the 10-nF filtering capacitor, however, are 21.23 and 86.10°. The probability of a short-term false detection is 8×10–5. From the properties of the cosine function, the sensitivity of the tilt detector rises with rising tilt angle. To select another value of tilt within this interval, you calculate the appropriate reference voltage from the equation cosαT=(VREF/VG) and then change the value of the RR resistors as necessary.

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 Figure 1).

References
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.


Caption
Figure 1: The tilt on MEMS accelerometer IC1 produces a voltage, VGZ. When compared with VREF and 2VREF, VGZ produces a digital output at the NAND gate.