mystery toy - uses photocells to play music when light changes

Hi, I have a kids toy that uses a photocell. When the photocell is covered up, it plays a sound. When the photocell is exposed to light again, the sound plays again.

How is this being done? I often would like to monitor a low-to-high or high-to-low transition on an analog pin for a microcontroller, but I don't want to keep reading the analog input; that would kill battery consumption. Is there a trick to translating the transition from low voltage (say ~.4V or less) to high voltage (~vcc-0.5), and turning that into a digital wake up from sleep?

I know some fancier microcontroller have ULP processors that can do analog pin reading capabilities. But I'm interested in learning a way to do this with hardware.

Thanks!

I am not sure where the toy has anything to do with anything. Can you simply state your objective without using the toy as an example ?

to save power. * Run MPU @ 1Mhz * use the sleep function

Which MPU ? The Arduino clock speed is configurable . I have never used other clock speeds with the UNO but have used many different clock speeds with ATtiny85s. Did you look at this?

or

this ?

I would use a micropower op amp as a comparator to monitor the photocell voltage divider, and wake up the processor when a certain light level is reached.

to save power. * Run MPU @ 1Mhz * use the sleep function

Are you talking about "hacking " the toy ?

Maybe a schmitt trigger ckt would work.

Just use a capacitor to change a level change into a pulse.

Hi, Configure the LDR potential divider to be below logic 0 threshold volts with dark and above logic 1 threshold volts when light.

Tom... :)