It’s possible that it was [Matt Meerian]’s awesome pun that won us over, not his ultrasonic bicycle dog defense system, but that would be silly. [Matt] wanted an elegant solution to a common problem when riding a bicycle, dogs. While, obscenities, ammonia, water, pepper spray, and others were suggested, they all had cons that just didn’t appeal to [Matt]. He liked the idea of using C02 powered high pressure sound waves to chase the dogs away with, but decided to choose a more electronic approach. He used a Atmel ATmega644 as the MCU, four 25kHz transmitters, and two 40kHz transmitters. When the rider sees a dog he simply flips a switch and it activates the transducers (along with, cleverly, a human audible horn so he doesn’t have to look down to know it’s working). So far [Matt] has not had a dog chase him in order to test it’s efficacy, but his cat clearly seems unaffected by the device as you can see after the break.