Quite a few people have asked questions about Pokey's flame sensor (the big, cylindrical yellow and black thing in the picture). Here's the rundown.
First and foremost, the design uses infrared LEDs for sensing infrared radiation. Yes, that's right, LEDs. Of course LEDs emit radiation when voltage is applied, but a little known fact is that they also generate a voltage in the presence of radiation -- and specifically radiation in the frequency range that they emit. I tried UV LEDs but they weren't sensitive enough to detect UV output of a candle. The pile of IR LEDs I ordered happened to generate about 0.5V max when pointed at incandescent lamps, candles, etc.
The only problem? Range. By themselves IR LEDs are a bit too limited in range to provide a strong signal from across the largest of firefighting rooms. I had the idea of using some kind of optics to increase range and after some experimentation the solution was to use the reflector off of a $3.50 Eveready lantern flashlight with the LED replacing the bulb. Range was increased to at least 8' -- plenty.
This device has the added benefit of limiting field of view, making the sensor less sensitive to IR sources outside the arena, but very sensitive to candles within the regulation height range. I installed dual IR LEDs in the housing hoping to give the sensor directionality but this didn't work out. A single LED works just as well.
Finally, to give the MCU's Analog to Digital Converter (ADC) a little more voltage range to work with, the signal from each LED runs through half of a simple, single-source op amp amplifier (LM258 or equiv) as pictured above. The resistor and capacitor in parallel seemed to help stabilize the LED signal when I was experimenting, taking ADC readings directly from the LED.
I used aluminum tape to block off the sensor's view to the side, essentially improving the "peakiness" of the sensor's response when scanning a room, making it easier to pinpoint the direction of the candle.
Edit: Brij asked why not use an IR phototransistor or photodiode (see comments). At the start, I tried several different sensors, actually. The phototransistor was too sensitive; pegged out too easily. Didn't think to try a photodiode. The LEDs showed a fairly linear response, just enough sensitivity. In short, they work great!