Yesterday I designed and built a temperature alarm system. It’s nothing too clever, just three large LED displays in a box, with an 18F25K22 PIC running everything. A Dallas 18B20 sensor has been mounted in a small box and is connected via a piece of three-core cable to the main display unit that can be mounted outside the room.
You can set the maximum allowed temperature and this is stored in the PIC’s EEPROM.
If the temperature rises about the pre-set maximum, the display flashes and there are a couple of 5v buzzers inside the main unit to help attract attention. There are three additional indicator LEDs housed within the main display unit as well. One is a green LED that flashes indicating all is well, a yellow LED that indicates that the unit is in setup-mode (started by powering on the unit whilst depressing the external push-button switch mounted on the side), and a red LED that indicates the temperature alarm has been tripped at some point. Depressing the push-button switch once clears this indicator and the complete unit is powered from a small 9v plug-in mains transformer.
Inside the unit there are two PCBs. One is the top display board, and the other is the logic and PSU board.
If people are interested I’ll publish the design details for this unit.