Sensors

Sensors convert physical quantities into electrical signals. Temperature, pressure, light, motion, humidity, distance, acceleration — if you can measure it, there's a sensor for it. The output is usually a voltage, current, resistance change, frequency, or digital protocol, which a microcontroller reads via ADC or a serial bus. Knowing what kind of output a sensor produces determines how to interface it, and knowing what accuracy and response speed you need narrows down which sensor family to use.

Sensor Output Types

Output type	Interface method	Examples	Notes
Analog voltage	ADC pin	LM35, TMP36, MQ-series gas sensors	Match sensor output range to ADC reference (0–3.3V, 0–5V)
Resistance change	Voltage divider + ADC	NTC thermistor, LDR, strain gauge, FSR	Choose divider resistor ≈ sensor mid-range value for best sensitivity
Current (4–20mA)	Shunt resistor + ADC or current receiver IC	Industrial pressure transducers, flow meters	4mA = zero, 20mA = full scale; loop powered; immune to voltage drop over long cable
Frequency / PWM	Timer input capture or interrupt	Flow sensors, tachometers, DHT22 (RH)	Count edges or measure period with MCU timer
I2C digital	SDA/SCL with 4.7kΩ pull-ups	DS18B20, BMP280, BME280, MPU-6050	Multi-device on one bus; address must be unique per device
SPI digital	MOSI, MISO, SCK, CS	MAX31855, ADXL345, MS5611	Faster than I2C, one CS line per device
1-Wire	Single data pin + 4.7kΩ pull-up to VCC	DS18B20, DS2401	Up to 127 devices on one wire; parasite power mode possible
UART serial	RX/TX at specific baud rate	GPS modules, LoRa modules, PM2.5 sensors	Fixed baud rate in datasheet; often 9600 or 115200

Temperature Sensors

Type	Range	Accuracy	Interface	Notes
NTC Thermistor	−50 to 150°C	±1–3°C	Voltage divider + ADC	Non-linear — needs Steinhart-Hart or lookup table. Very sensitive near nominal temp.
LM35 / TMP36	−40 to 150°C	±1°C	Analog voltage (10mV/°C)	Linear, calibrated, directly readable by ADC. Easy. Lower accuracy than digital.
DS18B20	−55 to 125°C	±0.5°C	1-Wire digital	The default choice for most hobbyist/maker work. Cheap, accurate, parasitic power mode.
BME280	−40 to 85°C	±1°C	I2C / SPI	Also measures humidity (±3%) and pressure (±1 hPa). One IC for three parameters.
Type K thermocouple + MAX31855	−200 to 1350°C	±2°C	SPI (via MAX31855)	For high temperatures that silicon sensors can't handle. Cold junction compensation in the IC.
Pt100 / Pt1000 RTD	−200 to 600°C	±0.1°C	Wheatstone bridge + ADC	Platinum resistance thermometers — highest accuracy, used in industrial/laboratory work

Thermistor Voltage Divider

Thermistor voltage divider: a fixed resistor and NTC thermistor create a temperature-dependent V_out for the ADC.

V_out = VCC × NTC_resistance / (R_fixed + NTC_resistance)

Best R_fixed = NTC value at your target/midpoint temperature
(maximises sensitivity at that temperature)

At 25°C with 10kΩ NTC: use R_fixed = 10kΩ
ADC reading → calculate NTC resistance → Steinhart-Hart → temperature

Light and Optical Sensors

Type	What it measures	Speed	Interface	Notes
LDR (photoresistor)	Light level (rough)	Slow (10–100ms)	Voltage divider + ADC	Non-linear, imprecise. Only use for light/dark detection — not measurement.
Photodiode	Light intensity (broad)	Fast (ns–µs)	Transimpedance amp + ADC	Low dark current, fast response. Needs op-amp to convert current to voltage.
Phototransistor	Light level	Moderate (µs)	Collector resistor + ADC	More sensitive than photodiode but slower. Used in optocouplers.
TSL2561 / BH1750	Lux (calibrated)	Moderate	I2C	Digital output in lux. Temperature-compensated. Use these when you need real light level.
VEML7700	Lux, white light	Moderate	I2C	16-bit, auto-gain, very low light capable. Better dynamic range than TSL2561.
TCS3200 / TCS34725	RGB colour	Moderate	Frequency / I2C	RGB + Clear channels. Used in colour sorting, white balance.

Motion and Position Sensors

Sensor	Measures	Interface	Notes
ADXL345	3-axis acceleration ±2–16g	I2C / SPI	Mature, well-documented, good for tilt sensing and vibration
MPU-6050	3-axis accel + 3-axis gyro	I2C	6-DOF IMU with internal DMP for sensor fusion. Ubiquitous in drone/flight controllers.
ICM-42688-P	6-DOF IMU (high performance)	SPI / I2C	Lower noise than MPU-6050, InvenSense successor
HC-SR501 PIR	Infrared motion (passive)	Digital HIGH/LOW	Detects warm moving objects. Adjustable sensitivity and hold time. Very common in security/alarm.
HC-SR04	Distance (ultrasonic)	Trigger/Echo pulses	2–400cm, ±3mm. Sound-based, affected by soft/angled surfaces. Simple to use.
VL53L0X / VL53L1X	Distance (laser ToF)	I2C	Time-of-flight laser, 2–200cm. Works on all surfaces including glass. Better than HC-SR04 for robotics.
AS5600	Absolute magnetic angle	I2C / Analog	12-bit, 0–360°, contactless (Hall effect over magnet). Motor shaft angle sensing.

HC-SR04 Distance Calculation

Trigger: send 10µs HIGH pulse on TRIG pin
Echo: measure duration of HIGH pulse on ECHO pin

Distance (cm) = pulse_duration_µs / 58
Distance (mm) = pulse_duration_µs × 0.1724

Or: Distance = (pulse_duration × speed_of_sound) / 2
            = (pulse_µs × 0.000343m/µs) / 2

Maximum range: ~400cm (echo too weak beyond this)
Minimum range: ~2cm (pulse and echo overlap below this)
Beam angle: ~15–30° cone — surfaces must face sensor squarely

Environmental Sensors

Sensor	Measures	Range	Interface	Notes
BME280	Pressure, humidity, temperature	300–1100 hPa, 0–100%RH, −40–85°C	I2C / SPI	The default all-in-one environmental sensor. Well-supported.
BMP390	Pressure, temperature	300–1250 hPa	I2C / SPI	Higher accuracy and lower noise than BME280 for altitude sensing
SHT31 / SHT40	Humidity + temperature	0–100%RH, −40–125°C	I2C	Better RH accuracy (±2%) than BME280 (±3%). Sensirion Swiss quality.
CCS811 / SGP30	eCO2, TVOC	ppm range	I2C	MOX gas sensors — need warm-up time, affected by humidity. For air quality indication.
SCD40 / SCD41	CO2 (true), humidity, temperature	0–40,000 ppm CO2	I2C	Photoacoustic CO2 measurement — true CO2, not eCO2 estimate. Sensirion.
MQ-2 / MQ-7 / MQ-135	Various gases (flammable, CO, air quality)	ppm range	Analog voltage	Cheap but require significant calibration. Output varies with temperature, humidity, and warm-up.

Analog Sensor Conditioning

Problem: sensor output is 0–100mV, MCU ADC is 0–3.3V
Solution: non-inverting op-amp amplifier

V_out = V_sensor × (1 + R_f/R_g)
Gain = 3.3/0.1 = 33 → R_f/R_g = 32 → use R_g=1kΩ, R_f=33kΩ

Problem: sensor has 4–20mA current output
Solution: shunt resistor (150Ω gives 0.6–3V) + optional op-amp buffer

At 4mA:  V = 0.004 × 150 = 0.6V (represents zero)
At 20mA: V = 0.020 × 150 = 3.0V (represents full scale)
Map 0.6–3.0V → 0–100% in MCU code

References

Horowitz & Hill — The Art of Electronics, 3rd ed. Chapters 15 (measurements)
Monk — Programming Arduino: Getting Started with Sketches
Adafruit Sensors — practical descriptions and libraries ↗
Sensirion — environmental sensor datasheets ↗