This is not homework, although I am an EE student. If the solution is cool/complex enough, this might become a design project, although it'll most likely end up as something to do over the summer. some time ago, I purchased a motorized doggie door. The dog wore a high frequency sound generator on it's collar, which triggered a sensor in the door when the dog was within a few feet. The chassis of the system is good, but the electronics sucked, and the company support was terrible, so I'm looking to gut and rebuild the innards. The system shouldn't be complicated, a microcontroller monitors a sensor, when the sensor kicks the MCU turns on the motor, when the sensor stops kicking the MCU turns off the motor and the weight of the door closes it (I think reverse-current protection was missing from the original design, which might help explain the short MTBF). I'm looking for ideas for a dog-detector that can: 1: detect the dog from about 3 feet away 2: work on both sides of the door (multiple antennas is acceptable) 3: not be too ugly that people will ask why there's a big coil of copper on the floor 4: should have a decent off-angle response, since my dog's not that bright, and won't always come straight at the door 5: detect my dog, but not my cats, or the local wildlife 6: no batteries on my dog, if possible RFID seems to be the best solution, but most sensors seem to be either very short range (10cm) or very long range (300ft) and very expensive. One of my classmates suggested a strong magnet on her collar, with an inductive coil on the floor, but this would violate rule 3, and the dog may end up with all kinds of stuff hanging from her collar. I'm not afraid of soldering, programming, or research, but I could use some bright ideas. Any suggestions?