I'm working on a device similar to the OpenROV, but with many more sensors, multi-day unattended deployment and no propulsion.
Since I've got no propulsion, I need someway to greatly increase buoyancy at the end of a deployment and a way to trigger this behavior from a distance.
I'm starting to work on a circuit based on the LM567 tone decoder chip. The idea is that a hydrophone will pick up a certain frequency and trigger a device to change buoyancy.
I have found a reasonably priced hydrophone, but I'm still trying to figure out how to generate an acoustic tone underwater.
Has anyone thought to do this with an ROV and if so, how do you generate the underwater sound?
I'm still working on how to change buoyancy but it will probably involve compressed air and an external bladder, or a ballast jettison system.