The device needed is a monitoring system. Aliens leave a probe on Earth or orbiting Earth, which typically lies dormant, but it awakes every 1,000 years or so to check life's status on Earth.
There's a clock hooked up to a pebble-bed reactor. The clock could take the form of a series of gears. Each subsequent gear will rotate at a slower rate than the previous gear. Once the last gear completes one revolution the pebble-bed reactor is triggered — pebbles are released from their repositories. The reactor then turns on and the monitoring devices are powered. Once a report has been generated, a signal containing it is broadcast to the alien's home star system. So, overall, the reactor runs for a short time. It then shuts down for 1,000 years until it's time for the system to send another report.
The reasoning for using a pebble-bed reactor is that the reactor depends on the pebbles' geometry. If the fuel pebbles aren't packed closely together, then the reactor is not critical, and fuel is not consumed — or consumed at a much lower rate.
It may take ~250 kilowatts for a data rate of 1000 bits per second per Project Longshot for a probe sending data from Alpha Centauri to Earth.