This page has a good explanation of a similar application: http://www.jrcomputing.com.au/Set_Watch/Set_Watch_Auto.html http://www.jrcomputing.com.au/Set_Watch/S

This page has a good explanation of a similar application: http://www.jrcomputing.com.au/Set_Watch/Set_Watch_Auto.html http://www.jrcomputing.com.au/Set_Watch/Set_Watch_Auto.html The app has the phone's audio output circuit send a modulated 20KHz tone to the headphones or speaker. This incidentally results in faint EM emission at the third harmonic (60KHz) which can be detected by the watch at short range. You can call this the "Part 15 loophole" [1]. [1]: https://www.gpo.gov/fdsys/pkg/CFR-2016-title47-vol1/xml/CFR-2016-title47-vol1-sec15-5.xml https://www.gpo.gov/fdsys/pkg/CFR-2016-title47-vol1/xml/CFR-...