If you want an extra challenge take a cellular module and try again. For example there is a GSM expansion for Raspberry Pi, try to build a "sensor" that can che

If you want an extra challenge take a cellular module and try again. For example there is a GSM expansion for Raspberry Pi, try to build a "sensor" that can check something every N seconds/minutes/hours and report back to you with a battery lasting months or more (hint- send it to sleep and wake on a timer)