It sounds to me as though you've got the basic idea.

Use a CDS cell or phototransister in a configuration where it outputs a
voltage corresponding to light level.

(simplified)
This voltage level goes two places
1) into a diode/capacitor high pass filter which then leads to an opamp
2) through a diode into the other opamp input

The output of the opamp is the derivative of the signal, ie how fast the
signal is changing.  A fast change (from light to dark) results in a
high voltage, where a slow change results in very little voltage.  A
change from dark to light is a high voltage in the opposite polarity.
 This can go directly to a schmidt trigger.

Saturation is not an issue if you pick the right components for the
light levels you're expecting, and even then it doesn't matter much.

If the signal is steady, the charge on the cap is equal to the signal
from the light sensor, and the opamp has a difference of 0 to multiply.
 This is the case whether the sensor is saturated or not.  If the sensor
is saturated, then the light can change dramatically within the range of
saturation and not trigger the device, but that is a simple problem of
determining reasonable light levels to design for.

-Adam




Giles Honeycutt wrote:

> 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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