Optical potentiometer (CB22159E)

This voltage-controlled potentiometer using an LDR was found in an English documentation. LDR is common and LED of any type.

Undervoltage protection (CB8028E)
Found in 1967 Raytheon documentation, this circuit uses a company coupler consisting of a lamp and...
Universal Crystal Test (CB21786E)
In a modern version this circuit can use an LED in series with a resistor in place of the lamp. The...
Simple Locked Alarm (CB18097E)
This circuit, when triggered, locks, and remains so until the source is switched off for a moment or...