Ultrasonic Pest Control
These Ultrasonic Pest Controls are often small boxes which are either battery powered or plugged into an electrical outlet. Some incorporate a PIR (Passive Infra Red) motion sensor which turns the device on when a body enters the controlled area. Not only does this save on the battery life, it also means that as animals associate the noise with their movement they are less likely to stop being afraid of the noise.
When selecting the correct Ultrasonic Pest Controller consider the range of the repeller, whether it incorporates a motion sensor and how it is powered - devices that are battery powered are very straight forward to install in your garden as they don't need to be plugged in. However, if you're using a repeller on a permanent basis and don't want the hassle of replacing the batteries, then a unit you can plug into your electric supply might be a better option.