Instructables

I need to build a light sensing circuit for a class project.

Hi,

I need to build a light sensing circuit for a class project. I have already built one circuit using a 741 opAmp chip, a 1 meg ohm photo cell, a 1 meg trimmer, a 1k, 100k, and a 47 ohm resister, and a 2n2222 transistor  as well as a 12vdc relay.  The circuit senses the light and the sensitivity can be adjusted by the potentiometer.  It works great for tuning it in to need a certain amount of light intensity to trigger the relay. The problem is i need to be able to put limiting factors on it.  I need to be able to set it to turn the relay on if a certain intensity is reached, but not turn on if the intensity gets to great.  Is there anyone who can help me with the schematics on how to achieve this?

Yes

Use two photo cells and hook it up as a comparator.
Never ask for help with an electronic problem with a verbal description of your circuit ! POST THE CIRCUIT.

Also, the key concept you need to look for is "Window Comparator"