This test circuit was found in the PE Hobbyst Handbook in a 1990s edition. The circuit tests the battery under load. The load, as the battery is selected by the switch.
General Electric suggests this circuit in a 1980s documentation. The operational of the time is...
This circuit, from a 1973 publication, locks the relay when the light that falls on the LDR is cut...
This circuit can be adapted to an FM radio or other receiver that does not have a tuning indicator LED....