This circuit was found in a Popular Electronics from the 90's. Its purpose is to detect the ionization of the air. The meter can be replaced by the current scale of an ordinary multimeter.
When the light falling on the sensor is cut off, the SCR fires, powering the lamp. Remember that...
When the sensor is wetted by a spray caused by someone falling into a pool or even by a wave...
This traditional TTL anti-clock setting was found in a 1993 American documentation. The circuit...