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Thanks for the great sketch. I had it working with a Uno, and I proceeded with "breadboarding" the Atmega328P MCU to further lower power consumption. I successfully bootloaded it to use the internal 8Mhz clock, and I can upload and run numerous simple sketches. But this one does not detect PIR motion anymore. Do you think the SLEEP_MODE_PWR_DOWN state does not work well with the 8Mhz internal clock?