Re: sensor circuit
I wanted to add something to the last post, because I am using a UV sensor for an application:
If you want to cover both ends of the visible spectrum then you can use UV and IR sensors i.e. if you want to verify the existance of a flame. Check some patents from www. freepatents.com to see how it works.
Furthermore, IanP said it all. What the nature of your application determines the sensor you need. If you want to combine them, then build a system that takes both measuremments and gives out a specific answer ( I don't know what) . If it is fire, then UV says flame, IR says also flame, you can check intensity of radiating energy in both spectrums and have an output that says ALARM, etc...
D.