Welcome new readers! Watch for articles to appear every 1-2 weeks, as if by magic. Podcasts appear... whenever I get around to it. On with the article:
While refitting Pokey the firefighting robot, I revisited the topic of candle flame detection with infrared LEDs, phototransistors, etc.
Originally, I'd used a pair of infrared (IR) light emitting diodes (LEDs) housed in a flashlight reflector housing. It was an inexpensive solution that seemed to have promise. It improved the sensitivity to distant/faint signals and reduced sensitivity to signals outside the regulation 15-20cm flame height.
I wasn't able to perfect the sensor and abandoned it in favor of a custom camera and software solution. Here's a summary of some information I collected along the way. perhaps it will help someone and inspire further investigation and experimentation.
Information originally collected from various Wikipedia articles.
Here's an excellent source of information.
Photodiode: Make use of the fact that LEDs have inherent capacitance. Charge the LED with the MCU interface pin set to output mode high. Then set the MCU interface pin to high impedance CMOS mode, and measure the time it takes for the LED to discharge. The rate of current leakage from the LED is proportional to the ambient light.
Photovoltaic mode: This is the mode I used for Pokey's sensor. Same concept as solar cells which, in fact, are just large photodiodes. Ambient light causes voltage to build up in the device. More voltage means more ambient light. I amplified the voltage with an operational amplifier and read it with an analog to digital converter.
Photoconductive mode: reverse bias the diode, decreasing inherent capacitance. Benefit is faster response times with the tradeoff of increased noise.
Avalanche photodiodes: similar to regular photodiodes but operated with higher reverse bias. Internal gain in the device comes from multiplication by avalanche breakdown.
Phototransistors: basically bipolar junction transistors with clear packaging such that photons reach the base-collector junction. The b-c junction acts as a photodiode whose current is amplified by the transistor's current gain (hfe). No better at detecting low light levels but more input-output gain (responsivity).