Anyone have a good voltage sensor they use?

I'm currently using this one:

But it does not seem to be picking up a voltage change where I am measuring. I'm measuring the voltage change which occurs on a light-sensor.
The light-sensor detects if there is light (or not). I'm hacking a piece of hardware, trying to wire in
to the existing light-sensor. I would also like some recommend on other light-sensors which I might
be able to replace the existing one with that would work with my application.

-= The Space Cowboy =-