Temperature

Fourteen ways to measure temperature, and the choice matters more than the count suggests: a chip that reads to a hundredth of a degree is useless on a furnace, and a thermocouple converter is pointless in a fridge.

Choosing

You are measuring	Reach for
room, cabinet, comfort range	an I2C sensor — `TMP102`, `LM75B`, `MCP9808`
something that must be accurate	`TMP117` (±0.1 °C) or `MCP9808` (±0.25 °C)
a furnace, a flue, a mould	a thermocouple — `MAX31855`, `MAX31856`, `MCP9600`
an industrial process with a PT100	`MAX31865`
something moving, or too hot to touch	`MLX90614` — infrared, non-contact

Rule of thumb. Below about 125 °C, an I2C sensor is cheaper, simpler and more accurate. Above it, you need a thermocouple or an RTD, and the extra wiring comes with them.

The traps

Address collisions

LM75B and TMP102 share the 0x48–0x4F range, and so do several others. Two chips on one bus at one address means neither works — and the symptom is nonsense readings, not silence. The project checks this when you build, so you find out at your desk.

Cold-junction compensation is not optional

A thermocouple measures a difference. The converter has to know its own temperature to give you an absolute one, which is why these chips report two channels. Put the converter somewhere with a stable temperature — not next to the heater it is measuring.

A PT100 wants to know how it is wired

MAX31865 needs telling whether the sensor is 2-, 3- or 4-wire and what the reference resistor is. Get it wrong and the reading is consistently off — often by a plausible-looking amount, which is the kind of error that survives for months.

The chips

Temperature sensors

Chip	Bus	What it is
`LM75B`	I2C	I2C temperature sensor (industry classic) — channel 0 = temperature [°C], 11-bit 0.125 °C resolution, address 0x48–0x4F. Collides with TMP102 addresses — packbuild watches the bus.
`MCP9808`	I2C	I2C precision temperature sensor ±0.25 °C — channel 0 = temperature [°C], 0.0625 °C resolution, address 0x18–0x1F.
`MLX90614`	I2C	I2C infrared thermometer (SMBus) — channels: 0 = object temperature [°C] (non-contact), 1 = ambient [°C]. Fixed address 0x5A. Bearings, contacts, moving parts.
`STS3X`	I2C	I2C temperature sensor (the SHT3x without the humidity cell) — channel 0 = temperature [°C]. Single shot with CRC. Address 0x4A–0x4B.
`STTS22H`	I2C	I2C temperature sensor (ST) — channel 0 = temperature [°C], 0.01 °C resolution, free-running 1 Hz. Address 0x38–0x3F by the ADDR pin. VERIFY ON THE BENCH (control register written from the datasheet, not tried on silicon).
`TC74`	I2C	I2C temperature sensor (single byte, 1 °C resolution) — channel 0 = temperature [°C]. Address 0x48–0x4F by suffix.
`TMP102`	I2C	I2C temperature sensor — channel 0 = temperature [°C], 12-bit 0.0625 °C resolution, address 0x48–0x4B.
`TMP117`	I2C	I2C precision temperature sensor ±0.1 °C — channel 0 = temperature [°C], 7.8125 m°C resolution, address 0x48–0x4B.

Thermocouple / RTD

Chip	Bus	What it is
`KMETERISO`	I2C	I2C isolated K-thermocouple module (M5Stack KMeterISO) — channel 0 = temperature [°C]. Fixed address 0x66. VERIFY ON THE BENCH.
`MAX31855`	SPI	SPI K-thermocouple converter — channels: 0 = thermocouple [°C] (0.25 °C steps), 1 = cold junction [°C]. Fault bits (open/short) drop the sample and log a diag entry. addr = CS pin.
`MAX31856`	SPI	SPI precision thermocouple converter (K type configured) — channels: 0 = thermocouple [°C] (19-bit, 0.0078 °C), 1 = cold junction [°C]. Fault register checked. addr = CS pin.
`MAX31865`	SPI	SPI RTD converter (PT100/PT1000) — °C straight into the variable. addr = CS pin. PeriphRec.config: bit0 = 3-wire, bit2 = 60 Hz filter (default 50 Hz), bit3 = PT1000, bits 4+ = Rref in tens of ohms (0 = 430 Ω for PT100 / 4300 Ω for PT1000 — common boards). Calibration via variable gain/offset; sensor fault = diag entry and the variable keeps its last value.
`MAX6675`	SPI	SPI K-thermocouple converter — channel 0 = temperature [°C], 0.25 °C steps, open-thermocouple detection. addr = CS pin.
`MCP9600`	I2C	I2C thermocouple converter (K type by default) — channel 0 = hot-junction temperature [°C], 0.0625 °C steps. Address 0x60–0x67.

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.