Acoustic Trigger and ballast jettison?


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.


Short version: You need a transducer and an oscillator.

Longer version: I once built a simple acoustic modem link* based on the LM567 circuit (or possibly a very similar one, this is some years ago but the name rings a bell). I used an piezo-electric acoustic transducer in both ends of the link. This is a relatively cheap, solid state component. To transmit a signal, I used a digital output of a microcontroller to generate the correct frequency. It could have been done smoother with more external circuitry, but this for this application weight and size was extremely critical.

(*) For those interested, this was a kind of pulse position modulation based link. Simple and robust, I guess it could work well for low bandwith control signals etc, while you might want a phase shift keyed system for higher bandwiths.