I have a small artificial tree named Douglas Fir which does about the same. Most use a small PIR (Passive InfraRed) sensor to detect motion. Sometimes you can just get a sensor bare bones or sometimes they come as a module. No clue what you have in there as to an actual circuit or schematic. You can try opening it up and get to the circuit board level. Try a Google of PIR sensor and look for anything which looks familiar. Could also be any of the other components in there. Less knowing exactly what you have it's about impossible to guess or suggest a specific part.
Ron