That's a tough one. I really think you'll have to amplify that signal in order to affect any kind of semiconductor switch. But there are some extremely low power opamps out there that you could use as an amplifier

Page 29 of this application note has a very simple photodiode amplifier circuit.

The op-amp they've used is an old one (it's an old app. note!) - there are better ones around these days, and almost anything will do.

You can use the op-amp output as one input to an LM311 comparator. The output of the comparator can directly drive a small relay (up to 50mA @ up to 36V).

Using an amp was something I thought of and to me seemed the only way to boost the output from the photodiode in order to get the circuit to turn on. The only problem is the amp would consume power; and I've been told to find a way without consuming any power. Something else I forgot to mention was that the rise time needs to be in the microseconds range and unfortunately solar cells and photoresistors are too slow.

Have you considered wiring several photodiodes in series to get the required output?

There are some extremely low power op amps available, the ISL28194 for example has a supply current of 0.33 microamps which at 5V would only use 1.65 microwatts of power.

We use optoisolators from Avago. The output stage is a transistor, and they have a photodiode from the base of the transistor to Vcc. Presumably the photodiode only draws current when light hits it.

Go to www.avagotech.com and do a search for 4n55, look at the figure on page 2 of the datasheet and you'll see the circuit I'm talking about (I can't paste from the pdf). It might be just what you need.

Thank you bhankiii, I will definitely take a look.

These guys make FET's with a zero volts gate turn on threshold. They are high impedance gates so they won't load your photodiode. You can buy them from DigiKey.

http://www.aldinc.com/

Thank you!