Found in an Electronic Design from the 80's, this circuit monitors a three-phase line, activating a relay when one of the phases is missing.
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...