How can ultraviolet and IR in flame detected?

Question

need a simple circuits to detect fire.

Answer 1

Perhaps try using photo-diodes/photo-transistors. different ones respond to different frequencies of light. I know you can get IR ones (like those in the receiver for TV-remotes). Think you can get ultraviolet ones too.

http://en.wikipedia.org/wiki/Photodiode

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Wire the diode/transistor in series with a resistor, and use it as a voltage divider. When light (of the appropriate range) shines on the semiconductor device, it's resistance will fall, altering the voltage of connection between them.

http://en.wikipedia.org/wiki/Voltage_div...

Most photo-diodes pass some current even  when they're not exposed to light, and you'll probably need to do some tweaking of values to get the circuit to detect fire correctly. Use a potentiometer as a voltage divider, to set a reference value. Use a comparator to compare the voltages coming from the voltage dividers. When one value is higher than the other you'll get an output signal. Swap the comparator inputs if you want the output to be inverted.

An op-amp takes the voltage at the "+ input", and subtracts the voltage at the "- input" from it. It then (ideally) multiples that by a huge amount (the gain) and tries to output it. It can't go above/bellow the supply voltage, so it usually ends up clipped to max/min values. When the two voltages are the same the output voltage from an op-amp lies half way between it power connections.

A comparator behaves just the same, except it NEVER outputs negative voltage (so requires a much simpler power supply)