We have them at our yard. Everyone has a key code given to them when they first arrive. You are not meant to pass this code on to anyone although people do shout them out from time to time. The farriers and vets have their own code (I believe). The YO can then see who has come on the yard when. This also prevents unauthorised people entering the yard.
You can key in the code from a horse box window as well as bending down when you are sat on a 17.1hh horse!
When they go wrong they are a total nightmare though and we have been locked in the yard before now when teh electrics have gone!
Horses soon get used to the noise they make, which is minimal anyway.