This car battery monitor was featured in the January 1988 issue of Radio Electronics magazine. The circuit uses common transistors and has 3 levels of indication made with common LEDs.
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....