Electronics notes/Movement sensing
Accelerometers and gyroscopes
An accelerometer measures linear force, in one or more directions.
And, since gravity is a force in a fixed direction, any slow-moving system (fast acceleration might saturate the sensor) can have at least some idea of its orientation - think pitch and roll (but not yaw).
While small, convenient, and cheap, they are neither the most sensitive or the lowest-noise. As such, they are often mostly useful for stronger motion, and most are specced for at least 1g.
Now typically MEMS designs: microelectromechanical - a tiny weight in a chip.
A gyroscope measures rotation,
ADXL345
A 3-axis accelerometer.
Speaks both I2C or SPI (1.6MHz or less) though breakout boards may expose only one.
Minimal pins:
- Gnd
- Vs (2.5V)
- Vddio (1.8V)
- 2/3 pins for I2C, 3/4 pins for SPI
Optional:
- INT1 and INT2 - can be used to signal:
- DATA_READY - watermark interrupt, signalling that the FIFO is full (in relevant FIFO modes)
- SINGLE_TAP
- DOUBLE_TAP
- FREE_FALL
Data output - there's a FIFO with a few modes:
- bypass mode: FIFO not used
- FIFO mode
- stops storing samples and waits for user to empty. Interrupt Tap/freefall detection still works.
- stream mode:
- overwrites oldest continuously
- trigger mode:
- only starts sampling when told to - behaving like FIFO mode.
- to reset,
Reading data
from DATAX, DATAY, DATAZ registers (two bytes each)
...in FIFO, stream, or trigger mode
A watermark bit signals whether the FIFO is full (stream or FIFO mode).
Can be configured as an interrupt.
MPU-6050
MPU-6050
3-axis gyroscope and 3-axis accelerometer plus a processor that does sensor fusion
Can also accept compass data into its sensor fusion(verify)
speed/precision tradeoff is configurable for both gyroscope and accelerometer
MPU-9150 - essentially a MPU-6050 plus an 3-axis magnetometer compass (AK8975).
...which can be used to reduce yaw drift.
https://invensense.tdk.com/wp-content/uploads/2015/02/MPU-6000-Datasheet1.pdf
https://invensense.tdk.com/wp-content/uploads/2015/02/MPU-9150-Datasheet.pdf
Seismological equipment
Variant naming comes largely from sensitivity (often in units of Volts/(meter/second)).
From geophone (relatively local) to local geologic seismographs to teleseismographs (world-scale) are factors of (very) roughly ten.
Geophone
A geopheone is essentially a voice coil, i.e. coil on an thing that can move, mounted to sense movement.
Many have a significant response peak, because of a spring that holds them and makes it a resonator.
They may have a relatively flat response curve in a wider band around that.
Since they're often used for earthquake detection, these frequencies are on the order of 5Hz, 10Hz, 15Hz. This type of design is impractical to design for below ~1Hz, though.
Hobbyists can get a decent signal quickly by connecting them directly to a (differential) ADC.
More serious uses would build an in-amp circuit, probably with bandpass.
DIY note: A torn woofer can be made into a geophone with some creativity, see e.g. https://zeidigital.wordpress.com/2015/08/14/base-for-earthquake-detector-geophone/
Seismometer
Potentially designed to detect as low as hundredths of a Hz
Unsorted
http://www.infiltec.com/seismo/
Radar
RCWL-0516
Effectively a motion sensor based on radar doppler (at ~3.1GHz).
Default sensitivity should work decently up to 8m, can be reduced.
Works through thin materials.
Unpopulated board positions:
- CDS - you can attach a cadmium disulphide sensor, which will pull that same line down when it is light
- useful for 'turn light on only in the dark' use
- R-CDS - add photoresistor to only work at low light
- C-TM - add capacitor to extend trigger time from default ~2s
- R-GN - add resistor to lower sensitivity(/distance)
- default is ~7m, adding 1M resistor makes it 5m(verify)