The main base station (Master) needs to be able to time the delay between an IR flash and the arrival of an ultrasound pulse (my best idea for estimating distance). I was thinking of offloading some of the work to a phone by using the headphone jack as IO. However, the post on teensies made me wonder if detecting a 25kHz tome would be doable fast enough on a low cost arm processor. This would make the project more self-contained and probably cheaper... Anyone have thoughts on this, or experience detecting specific frequencies with a micro?