Wanted: Simple Light Detector Circuit

I need a sensitive light detector that i can place in front of the eyepiece
of
a binocular. This detector should sound a "beep" whenever it detects light.
I would also like it to be powered by 6 vdc or less.

 

What amount of light? A star bright star in view? Headlights 50 feet
away shining directly on the binoculars? Sunlight? That covers a
range of millions to one, I think.

 

I should have been specific....."Starlight" is what i want to detect.

 

The basic photo diode amplifier is shown on page 6 of this opamp
reference:
http://www.national.com/an/AN/AN-20.pdf

If you connect a second opamp as a comparator you can drive a small
piezo buzzer directly from the second opamp. Since opamps amplify the
difference between their two inputs, you connect the output of the
first amplifier to one of the second one's inputs and connect the
other input to a trim pot connected as a supply voltage divider that
can produce any voltage between the positive and negative supply
rails. I think I would start with a pair of 9 volt batteries
connected in series as the supply, (18 volts total) with the junction
between them as the signal ground. An LM358 dual opamp might be good
enough and is very common.

Here are the data sheets for some photo diodes sold through Digikey.
http://rocky.digikey.com/WebLib/Sharp/Web Data/bs520.pdf
http://www.photonicdetectors.com/pdf/pdbc107.pdf
http://www.photonicdetectors.com/pdf/pdbc113.pdf
http://www.photonicdetectors.com/pdf/pdbc140.pdf
http://rocky.digikey.com/WebLib/Photonic Detectors/Web Data/pdbc135.pdf

Here is an example of a piezo buzzer that an opamp can drive:
http://rocky.digikey.com/WebLib/CUI Inc/Web data/CEP-2260A.pdf

You may need an additional lens to form an image of the star on the
detector like your eye's lens does for your retina.