This circuit is intended for underwater communications. Of course, the critical point is the shielding to avoid water. The circuit is from an Electronic Hobbysts Handbook from the 90s.
This is a combined circuit of tone generator and preamplifier. We found it in a 1975...
We found this simple alarm circuit in a 1986 documentation. The circuit uses sensors of the NA...
We found this circuit in the Linear Applications Handbook by National Semiconductor. The base is...