The Contech Scarecrow is a motion activated water spray dog repeller. To install you must connect it to your garden hose, when a dog passes infront of the detector the repeller lets out a quick water jet to scare them away. These can be very effective against dogs, though the effectiveness will vary depending on the breed of dog. The range of the spray is 1-3m (depending on your water pressure) so less than half the range an ultrasonic repeller. It runs off a 9V battery.
Water repellers are effective against many other animals including rabbits, foxes and cats. If you have tried an ultrasonic repeller with out success then this water repeller is a certain solution. The downside being the need for the hose attachment and the risk of being sprayed yourself.
Be aware that you will need at least 2 bar pressure from your water hose to have a good range. Also water repellers are more suited to the summer months, in bad winters water freezing may become a problem.