First of all, tell me where this Engel sub is, I wouldn't mind it as a Christmas present...

Secondly,

Good idea, but may I suggest this method as well?

The limpet mines in the war used to work with a sweet. The sweet would hold apart two spring loaded pins and when the sweet dissolved the pins would touch completing the circuit.

I know it isn't technically loss of signal, but it works just as well providing you are willing to replace the sweet.

Michael