Wireless "Arduino" sensors - what is at least needed on circuit?
I designed a transmit-only device for use in a big freezer department in a store. They used 433MHz and a cheap little PIC micro that woke up about once per minute and transmitted temperature and its battery level to a central collection point for all the local devices. The wake-up time between one unit and the next was about a minute with a bit of variability due to their unique ID codes.

This meant the central receiving unit sometimes got garbage but, in the main it could pick up all the data from up to 100 (I think it was 100) units in about 5 minutes (given collisions were not infrequent.

Battery lasted about 1 year.

The up-time was about 40ms given that the crystal needed about 20ms to stabilize and I suspect that should I have used an RC clock I could have halved this time. The transmission was: -


Preamble to lock the central receivers data slicer
Header with ID and battery status
Temperature
16-bit checksum


Hope this helps. Here are a couple of little circuits to get you on your way. They use PICs but the general functionality can be transferred to virtually any MCU: -



They came from here