Distance and Proximity Sensors

Ultrasonic (HC-SR04): sends a 40 kHz burst and times the echo:

$d = \frac{v_{sound}\, t}{2} \approx \frac{343 \times t}{2}\ \text{m}$

(divide by 2 because the sound goes there and back; speed of sound rises about 0.6 m/s per °C).

IR triangulation (Sharp GP2Y0A21): analog output, non-linear. Time-of-flight laser (VL53L0X/VL53L1X): millimetre-resolution distance over I2C up to a few metres. Capacitive and inductive proximity sensors detect nearby objects or metal without contact.