I want to try to detect individual photons of light. I have a BPX61 photodiode and an arduino. The light levels will be very low so I'll need to amplify the signal. I'm thinking a 2N2222 would do it. I'll be running the whole lot off a 5v power source. So far I'm not finding any usable circuit diagrams with resistor/capacitor values that would be suitable. Has anybody any good links?
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What might work depends on the speed of the signal.
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A voltage comparator or 5v Op amp might work for this.
You need more gain. Photodiode Amplifier.
I have no idea if you can detect an "individual photon". My guess is, not with anything homemade.
You can. It's one of the most famous experiments in quantum physics. Harvard University had an account of it on their website, but there's no detector circuit given so no help to the OP.
That's the thing... I don't have any opamp or voltage comparator chips. I do have transistors, capacitors and resistors. If I were to order chips then they would take weeks to arrive and I'd have to make up a minimum order quantity since Radio Shack went bust.
Use a photon counter. They are commercially available, usually based on avalanche PIN diodes (Silicon Photomultipliers).
The dark noise of an ordinary photodiode makes it impossible to detect individual photons.
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- There's the rub . . .
I don't have one. I do have basic electronic components.
Give it a try with what you have. It will be a good learning experience.
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How will you know when you have succeeded? How will you know if your detector actually detected 2 or 10 photons, not just 1?
Pretty much. You need a PMT, I don't think even the commonly-available avalanche diodes can detect single photons. And even then, you need pretty high speed electronics to read the PMT output.
Hamamatsu Avalanche Photodiodes | Edmund Optics
I've used Hamamatsu Photomultiplier Tubes before but with custom electronics. Looks like they have one that connects to USB, but it's spendy.
Hamamatsu USB-Powered Photon Counting PMT Modules | Edmund Optics
@OP: you're not going to be able to do this with just random stuff you have on hand. Unless you have a very well stocked junk pile 
Hi, @mysteriousorb
I think you will find that a photo-diode is too noisy to detect a single photon.
Think you need a type of Photo-multiplier device, but you need to be very careful as they are an extremely sensitive devices and can be damaged by normal light levels.
What is your overall project?
What do you need the counter for?
Tom.... 
No, because it cant reasonably be done like that.
Look at this
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