Distance sensor (HC-SR04)

Software

You can download the Arduino sketch here:
http://code.mios.com/trac/mios_arduino-sensor/browser/trunk/Arduino/DistanceSensor

Adjust the measure interval by updating SLEEP_TIME define at the top of the sketch file.

This sensor requires you to upload the following device files to Vera as well.
http://code.mios.com/trac/mios_arduino-sensor/browser/trunk/Vera
D_DistanceSensor1.xml
D_DistanceSensor1.json
S_DistanceSensor1.xml

Wire things up

Follow the instructions in ConnectingRadioModule to connect radio module.

The HC-SR04 sends out short ultrasonic pulses and registers when the sound echo comes back. By knowing the speed of sound you can easily calculate the distance. To connect the HC-SR04 sensor to the Arduino board follow these instructions:
Connect VCC of the HC-SR04 sensor to +5V
Connect Echo to digital pin 5 of the Arduino board
Connect Trig to digital pin 6 of the Arduino board
Connect GND to Arduino ground

Hardware - Purchase guide

To build this sensor you'll need one arduino + radio specified in the base kit found here:
http://code.mios.com/trac/mios_arduino-sensor/wiki/WikiStart#Buyingguide
With the following additional parts:

1x Distance Sensor HC-SR04
​http://www.aliexpress.com/item/Free-shipping-2pcs-Ultrasonic-Module-HC-SR04-Distance-Measuring-Transducer-Sensor-for-Arduino-Samples-Best-prices/690139020.html

TOTAL SENSOR PRICE: $7.6 (arduino + radio) + $2.42 (HC-SR04) = $10