This is an Arduino-based Proximity Sensor circuit wired using the popular HC-SR04 Ultrasonic ranging module,used here for non-contact object detection purpose. The HC-SR04 module includes ultrasonic ...

The system here uses a cheap Arduino board, a few sensors, and a magnetic contact switch. It’s then hooked into the Adafruit IO and connected to IFTTT, so you can set up alerts in any way you like.