OK, their is a 100 ways to do this, but I don't want to re-invent the wheel.
  Are their any reference designs for something like the crocking frog that
makes noise when someone walks in front of it?  Are they using a
photo-resistor and sensing change with a low pass filter?  Are they using a
photo diode? Photo Transistor?  If so, how do they avoid total saturation?

Best regards,
Giles

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