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.
In the figure we have the way to use a common LED as a light sensor. The operational power supply...
Found in Radio Electronics from the 90s, this circuit generates signals from 100 Hz to 1.5 kHz....
Found in a 1960 Sylvania manual, this circuit can be implemented with a modern equivalent...