Ultrasonic not PIR

Ultrasonic for the Win 

To detect motion in order to set off the warning 'growls', I have looked at using a PIR sensor. 

While I can use it, I have been advised by the world's best tutor, i.e. James (See James I am a nice person!) that it's not the best sensor to use and would basically be a massive pain trying to code and deal with its delicate sensibilities. To instead use an ultrasonic sensor to detect motion etc. They are easier to code and will get the job just as good.

Basically, the moral of the post, is to avoid PIR and always listen to your Tutors, they know what's best!