The fish finder is a self contained, highly rugged amplifier with speaker. It is designed to be waterproof and connected directly to a pair of electrodes that are attached at the end of a long pole which is then hand held. A neck strap allows for the device to be worn hands free so that the electrodes can be positioned in the water with the pole under foliage or rocks where fish might be hiding. An assistant can then use a net to catch the electric fish with a net.
The Fish Finder has been published in the UCF Undergraduate Research Journal Design Improvements and Construction of an Electric Fish Finder.
The FIsh Finder is also featured on a National Geographic TV episode. You can see the clip Catching Electric Eels where Dr. Crampton uses the Fish Finder to catch an electric eel.
Here's the schematic for the Fish Finder. It's a basic series of operational amplifiers.
Stage A is an instrumentation amplifier. Stage B is a gain stage. Depending upon the position of the power switch, stage C couples to either stage A or stage B. Stage C does have some gain, since it is designed to accept an audio line level input and output a low impedance signal to drive a speaker. Stage D is a TLE2426 which is an active virtual ground. I chose to use an active virtual ground as it enhances stability by lowering the impedance of the virtual ground as compared to a simple voltage divider (which can become unbalanced should a component draw excessive current).
The circuit is designed to operate on a 9V supply; we use a 6 cell 1.5V pack as it offers superior battery life over the 2 9 volts shown in the schematic.
The connector on the chassis is a high quality water proof Lemo connector with 3 pins. The most unreliable component on the design of the Fish Finder so far has been the switches which have failed due to damage caused by inward force of the switch toggle during transit.