Pressure and position

Ten pressure sensors and six position sensors.

Pressure

Two different jobs share the word.

Barometric — BMP280, BMP581, MS5611, LPS22, MPL3115A2 — measures air pressure: weather, altitude, or the drop across a filter. Fine enough to see a few metres of altitude change.

Process — HONEYWELLABP, HONEYWELLABP2, XGZP68XX, SDP3X — measures a fluid or a gas in a pipe or a duct, in the units a setup cares about. SDP3X measures differential pressure, which is what a filter monitor or an air-flow measurement actually needs.

You are measuring	Reach for
weather, altitude	`BMP280`, `MS5611`, `MPL3115A2`
pressure and humidity together	`MS8607`
a pipe or a tank	`HONEYWELLABP`, `XGZP68XX`
the difference across a filter	`SDP3X`
hydraulics at industrial pressures	a 4–20 mA transmitter into an analog input

Altitude from pressure needs a reference

Absolute pressure changes with the weather by more than it changes over several floors of a building. Altitude without a reference pressure drifts by tens of metres between a sunny day and a storm.

Position

Magnetic angle — AS5600 — reads the angle of a magnet on a shaft. No contact, no wear, full-turn resolution. A good answer for a rotating axis that does not need to count revolutions.

Compass — HMC5883L, QMC5883L — measures the earth's field, and therefore heading. On a setup, more often used to detect a large steel object moving nearby.

Inertial — MPU6050, BMI160, LSM6DS — accelerometer and gyroscope together: tilt, and vibration.

Vibration is easier than orientation

Getting a stable angle out of an inertial sensor needs filtering and care. Noticing that vibration has doubled since last month needs almost none — and in a real setup, that is usually the question worth asking. A bearing announces itself weeks before it fails.

A magnetic angle sensor cares where the magnet is

AS5600 wants the magnet centred over the chip, at the distance the datasheet gives, with no steel nearby. A millimetre of offset becomes degrees of error — and the error varies through the turn, which makes it look like the mechanics are at fault.

A compass in a steel enclosure measures the enclosure.

The chips

Pressure sensors

Chip	Bus	What it is
`BMP085`	I2C	I2C barometer (BMP085/BMP180, integer datasheet compensation) — channels: 0 = pressure [hPa], 1 = temperature [°C]. Fixed address 0x77.
`BMP581`	I2C	I2C pressure + temperature sensor (Bosch, direct-reading — no compensation math) — channels: 0 = pressure [hPa], 1 = temperature [°C]. Address 0x46–0x47. VERIFY ON THE BENCH.
`HONEYWELLABP`	I2C	I2C board-mount pressure sensor (Honeywell ABP series) — channels: 0 = raw bridge counts (10–90 % of 2^14; the range is ordering-specific, engineering units via variable gain/offset), 1 = temperature [°C] (11-bit). The address is ordering-specific too — the check accepts 0x08–0x78.
`HONEYWELLABP2`	I2C	I2C micropressure sensor (Honeywell ABP2/MPR series) — channels: 0 = raw 24-bit counts (units via variable gain/offset), 1 = temperature [°C]. Command-triggered conversion; the address is ordering-specific — the check accepts 0x08–0x78.
`LPS22`	I2C	I2C pressure + temperature sensor (ST LPS22HB/HH) — channels: 0 = pressure [hPa], 1 = temperature [°C]. 1 Hz continuous. Address 0x5C–0x5D.
`MPL3115A2`	I2C	I2C barometer/altimeter (NXP) — channels: 0 = pressure [hPa], 1 = temperature [°C]. Barometer mode, OSR 128. Fixed address 0x60.
`MS5611`	I2C	I2C barometer (PROM coefficients + first-order datasheet compensation) — channels: 0 = pressure [hPa], 1 = temperature [°C]. Address 0x76–0x77.
`MS8607`	I2C	I2C PHT combo, the pressure/temperature die (MS5637-class compensation) — channels: 0 = pressure [hPa], 1 = temperature [°C]. The humidity die is a separate HTU21D at 0x40 — add it as its own peripheral. Fixed address 0x76.
`SDP3X`	I2C	I2C differential pressure sensor (Sensirion SDP3x) — channels: 0 = differential pressure [Pa] (the chip reports its own scale factor), 1 = temperature [°C]. Continuous measurement with averaging; address 0x21–0x23. Filter and duct monitoring.
`XGZP68XX`	I2C	I2C pressure sensor (CFSensor XGZP68xx modules) — channels: 0 = raw 24-bit counts (the K factor of the ordering goes into variable gain), 1 = temperature [°C]. Fixed address 0x6D. VERIFY ON THE BENCH.

Position sensors

Chip	Bus	What it is
`AS5600`	I2C	I2C magnetic angle sensor (12-bit) — channels: 0 = raw angle counts 0–4095 (degrees via variable gain 360/4096), 1 = magnet detected (1/0). Fixed address 0x36 — a poor man's absolute encoder for slow axes and dampers.
`BMI160`	I2C	I2C accelerometer + gyroscope (Bosch) — channels: 0–2 = acceleration X/Y/Z [g], 3–5 = angular rate X/Y/Z [°/s], 6 = die temperature [°C]. Ranges ±2 g / ±2000 °/s. Address 0x68–0x69.
`HMC5883L`	I2C	I2C magnetometer (Honeywell classic) — channels: 0–2 = field X/Y/Z [gauss]. Fixed address 0x1E; continuous mode, ±1.3 G range.
`LSM6DS`	I2C	I2C accelerometer + gyroscope (ST LSM6DS3/DSO family) — channels: 0–2 = acceleration X/Y/Z [g], 3–5 = angular rate X/Y/Z [°/s], 6 = die temperature [°C]. Ranges ±2 g / ±250 °/s. Address 0x6A–0x6B.
`MPU6050`	I2C	I2C accelerometer + gyroscope — channels: 0–2 = acceleration X/Y/Z [g], 3–5 = angular rate X/Y/Z [°/s], 6 = die temperature [°C]. Ranges ±2 g / ±250 °/s. Address 0x68–0x69. Vibration and tilt monitoring.
`QMC5883L`	I2C	I2C magnetometer (the HMC5883L clone) — channels: 0–2 = field X/Y/Z [gauss]. Fixed address 0x0D; continuous mode, ±8 G range.

Every entry above is in the editor: pick the chip, give it an address or a chip-select pin, and its channels become variables. See Adding a peripheral for the click-through.