Distance sensor (HC-SR04)

Software

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

Before downloading the sketch to the Arduino you need to select a unique RADIO_ID (see top of sketch source code). The RADIO_ID must be unique in your radio network and is also used on the Vera side to distinguish between different sensors (1-255). After updating RADIO_ID and downloading the code you should be ready to go. Adjust the measure interval by updating SLEEP_TIME define at the top of the sketch file.

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

This is the stuff you need to purchase to build the HumiditySensor. Note that I've provided links to the things I bought during the development. You might find it cheaper somewhere else. Also note that you might need to buy like 10x of some items to get the price specified.

1x Cheap Arduino Nano without serial interface and usb connector
​http://www.aliexpress.com/item/16Pcs-Lot-Pro-Mini-Module-Atmega328-5V-16M-For-Arduino-Compatible-Nano/710613095.html

8x Dupont cable Female->Female for radio module (and you might need 4 extra cables for your distance sensor)
​http://www.aliexpress.com/item/New-Female-To-Female-40-pcs-Wire-Cable-Line-1p-1p-Pin-Connector-20cm-2-54mm/1045212486.html

1x NRF24L01+ wireless data transmission module 2.4Ghz
​http://www.aliexpress.com/item/Free-shipping-NRF24L01-wireless-data-transmission-module-2-4G-the-NRF24L01-5PCS-LOT-upgrade-version-New/603756080.html

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

(You might later need some housing and transformer or battery compartment as well)

TOTAL SENSOR PRICE: $3,99 + $0.93 + $0,6 + $2,42 = $7,94

To download code to the cheapest Arduino Nano sensors (that is missing the serial USB port) you need to buy one of following as well. It is only attached while downloading code and can be reused between sensors.

1x Serial to USB adapter to download code to cheap Arduino Nanos
​http://www.aliexpress.com/item/Free-Shipping-FT232RL-USB-To-Serial-Adapter-Module-USB-TO-232-For-Arduino-Download-Cable/827218126.html