Build an Arduino-controlled Temperature Alarm
You can do a lot with a little using Arduino and various shields, and with this sprit we love the temperature alarm described over a the tronixstuff.com website.
They've used an all-in-one LED real-time clock shield that also has an onboard thermistor to make a simple alarm that will alert the user when ambient temperature drops below or exceeds a certain temperature range. A quick demonstration is shown in the following video:
Operation of the shield and its features is described, along with a documented sketch for the alarm - so visit the tutorial to get started.