Would be the absolutely minimum useful circuit! I need this big pocket illuminating by night when I open it up, nothing fancy, just get some light and light up the LED. But the pocket is already heavy, so I am thinking of a cell button battery if at all. Only then got this idea that a photodiode could be use to provide some current, enough to wade inside for a better light. I was thinking after the two components circuit a resistor could be added to gauge sensitivity and maybe a capacitor to help things out, but as an absolute minimum a photodiode should be enough to turn light into current and drive the LED. Even a battery of photodiodes would be preferable than a battery for my application. I would be trying it out but I only have photoresistors at hand and the photodiode would take some time in reaching me.
No, not enough current.
Won't work: sorry, end of dream. 
Photodiodes provide very little current, way less than the little bit of current required by an LED. Besides, if you need an LED for light, where are you going to get the light for the photodiodes? And remember, there is no such thing as perpetual motion.
And remember, there is no such thing as perpetual motion.
Point the LED at the photodiode, and it will generate current to light the LED!
You can light an LED by shining light at other LEDs.
But it's just a neat trick, not a useful device.
Put this right up there with wind-powered fan.
The second law of thermodynamics basically says 'you can't win - by whatever route'
and hence no perpetual motion machine is possible.
so far as I know it hasn't been repealed yet
regards
Allan
The Three Laws of Thermodynamics, paraphrased:
- You can't win.
- You can't break even.
- You can't get out of the game!
Not perpetual motion but transforming and redirecting photons into current. So we do not have efficiently transforming photodiodes save solar cells... The pocket is quite dark even when exposed to light, I do have to shine something in to see deep inside. Anyway, I need whatever solution to illuminate only when I open the pocket and better be energy cheap and light. 
Doesn't get any cheaper than rechargeable batteries. Slip an LED on a rechargeable 2032, easy slip to break contacts when you don't need it.
I would be trying it out but I only have photoresistors at hand and the photodiode would take some time in reaching me.
Maybe an LED could drive itself.
syntotic:
Would be the absolutely minimum useful circuit! I need this big pocket illuminating by night when I open it up, nothing fancy, just get some light and light up the LED. But the pocket is already heavy, so I am thinking of a cell button battery if at all. Only then got this idea that a photodiode could be use to provide some current, enough to wade inside for a better light. I was thinking after the two components circuit a resistor could be added to gauge sensitivity and maybe a capacitor to help things out, but as an absolute minimum a photodiode should be enough to turn light into current and drive the LED. Even a battery of photodiodes would be preferable than a battery for my application. I would be trying it out but I only have photoresistors at hand and the photodiode would take some time in reaching me.
You appear to want to somehow generate light from light in the dark.
That makes no sense.
If your bag is so big, that you can't find stuff in the dark, the weight of 2 AAA-batteries probably won't hurt. Why exactly is it so important to be lightweight?
lg, couka
syntotic:
Not perpetual motion but transforming and redirecting photons into current. So we do not have efficiently transforming photodiodes save solar cells... The pocket is quite dark even when exposed to light, I do have to shine something in to see deep inside. Anyway, I need whatever solution to illuminate only when I open the pocket and better be energy cheap and light.
Transparent window in the pocket?!
Solar cells? At night?
So the goal is to redirect photons. Fiber optics.
No point in magically turning photons to current back to photons.
dlloyd:
Maybe an LED could drive itself.
Onley when taught at an early age
He he, I will try glow-in-the-night-radioactive-silly putty! Easier than electronics.
Optic fiber, did not think of it, but it will solve a problem or two, too.
Really, a button cell battery, LED, photoresistor should be a good minimum throwaway circuit to start illuminating as soon as the pocket gets the light that it was opened.
Just a 2032 button cell and an LED can get 10 straight days of useful light being constantly on. This is my firsthand experience. Put a LDR in series with that and maybe it'll do what you want. Even though it sounds weird, needing light to turn on a light, but if the LDR picks up enough ambient light near the top of this compartment, the LED can light it up better.
You can actually buy light emitting cells basded on tritium and a phosphor - night carp fishermen use them..they're pretty bright too. I think they're called 'betalights'.....
regards
Allan
Tritium vials are expensive and not useful as illumination lighting