Found in a 90s documentation, this circuit makes use of D1, a switching diode that is uncommon today. D3 is an LED. The circuit monitors 12 V batteries.
This circuit provides both frequency and voltage division. The circuit was found in a 1966...
This circuit converts a signal from any waveform into a rectangular or square signal of the same...
Found in Unitrode documentation from the 1980s, this circuit makes use of PUTs, currently common...