An Arduino is a microcontroller — a single board designed to control something ... an accelerometer for detecting motion, or ...

If you’re going to develop another Arduino-compatible board these days ... Stick the Smart Citizen somewhere and it can send sensor data to the web over a WiFi connection.

[mircemk] shows how to create a simple non-contact proximity sensor using little more than an Arduino Nano board, and a convenient software library intended to measure the value of capacitors.