I’d like some clever person to design a light/bell/computer that has a ‘dog’ mode. i.e. it emits a high pitched sound audible to dogs but not to humans. That assumes that it would warn dogs off, rather than bring them out to 'ave a go at my ankles… I really haven’t done any research. (Could have done with something similar when I worked as a postie for a while some years ago.)
In Colorado (and elsewhere, I’m sure) they sold little plastic whistle-type devices that you could mount on your front bumper. They were supposed to emit a high frequency from the wind whilst driving to ward off deer. Was to prevent them from jumping out in front of you on the mountain roads.
However, I’m pretty sure some people researched the efficacy of it and found they really didn’t work.