This is a small device I made over a weekend using some components off AliExpress (Chinese supplier) to locate my bike in the mazes of the Delft train station. Simply put, the bike makes a beeping noise when the button on the key chain is pressed. It has a quite long range (~50m) and receiver will only listen to this particular transmitter. It operates on the LPD433 band, just like a car fob.
The transmitter/receiver set was not documented well, so I used an oscilloscope to measure some of the characteristics of the set. The "time to boot" of the receiver turned out to be around 200 ms, and the receiver drained around 7 mA while listening. This gives you a very short run-time without any power management (1 wk off 3 AA batteries), so I made a model on Matlab to see how a smart power cycling scheme could extend the battery life.
The simulation shown that with the power cycling, the device would run for around a year off the same battery pack. I might implement this scheme in the future when I also have some spare low-power controllers. For now I just take a photo of the rack.