This 1967 Raytheon circuit uses a Raystor, an optical coupler with a lamp and an LDR. The circuit can be adapted to use another type of coupler.
This experimental amplifier was found in the 1960 Mullard transistor reference manual. The...
This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit...
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...