If you have been listening to security now with Steve Gibson for a few years you probably heard about the portable dog killer (listen from 57:46). Not sure if Matt Meerian has listened to this Security Now episode but the Ultrasonic Dog and Cat Deterrent Project he sent in sure sounds similar to that project. Matt built this project to use on a bike to ward off dogs and cars from getting in the path of the bike. As most of us know not all projects are successful (watch the video for some fun results).
“For the main MCU, I used an Atmel ATmega644. I needed an MCU with three timers for the different frequency outputs. Besides, I already had the MCU.  (the board in the picture above uses and ATMEGA32) When a dog is heard getting close, the rider flips a toggle switch under the seat on. The emitters then start doing their thing and hopefully drive the dog away. I put a human audible piezo emitter next to the toggle switch to let me know it is working. (this is much better than an LED, I would have to look at the LED, which would take my eyes off of the road and the dog)”