We found this circuit in an old RCA publication. The components are of the time, admitting equivalents. The circuit is triggered in two ways using LDRs.
This filter for the 300 Hz to 3 kHz range was found in an American documentation from the 90's....
The battery is charged by the photocell that keeps the low consumption beacon in the dark. The...
This is the typical circuit of an FET amplifier stage in the common source configuration. The circuit is...