The pinger chips are made available to AMS members only, they cost £5 each I believe. If you can code a microcontroller (PIC, Atmel), you can come up with something similar, the design emitts a swooping sound somewhat like an ascending square or triangle waveform, which sounds every few seconds. As time goes on the interval between swoops increases in order to preserve the life of the battery. The pinger can apparently last for several days, which gives you a sporting chance of arranging for a diver if the model goes too deep to swim for it yourself. Details on how to build the hydrophone amplifier are published on their forum, it also shows a built up pinger which uses a PIC microcontroller coupled to a piezo mounted in a small capsule (plumbing fittings) with a self contained power supply- two button cells. http://www.theassociationofmodelsubmari ... ydrophones
The piezo transducer is epoxied into the enclosure, which also acts as a resonant chamber. The unit can be mounted inside or outside the main dive module. For small models, mounting externally could really upset the balance and internal mounting will take up too much space. Alternatives could be using an endcap on a dive module and modifying it to create a small watertight area of its own.
The original link on how to build hydrophones seems to be dead, but here is another that should work well enough-http://maaheli.ee/main/archives/272