Do you need to use a Microcontroller or just sensors ?
You can make a "Heat detector" using the LM35 - Temperature Sensors.
I dont know anything about smoke detectors though but you can get a real smoke detector , open it and see how its working...
What is a COB? All you need is a comparator and something to drive the alarm, if the comparator is not able to drive it direct. I suggest you post your circuit here and ask for comments.