Ever wonder how those cheap devices at Walmart sense you walking by so they can play the cricket sound or meow like a cat? Have a look at the circuit for these Spooky Halloween Eyes for some details. A slight change in ambient light is all it takes for the small device to "see" that someone is close.
"The schematic is quite simple, each of the devices (LED’s, phototransistor, sounder) are simply connected to the appropriate uC pin. The phototransistor is biased by a single 499kohm resistor. This seemed to give a nice range of resulting voltages in the low light levels expected during operation. Triggering is done using by detecting a change in illumination, not an absolute threshold, thus this should provide nice performance."
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