Undervoltage detector with the LM3900 (CB25172E)

The figure shows how to implement an undervoltage detector using a Norton amplifier and a zener diode as the main elements.

Multi-pulse trigger (CB21477E)
This circuit was found in a 1967 EEE. The transistors are from the time and can be tested...
Zener diode expander (CB8819E)
With this circuit it is possible to obtain a higher voltage from a zener diode. The circuit is from...
Transistor Replacement Box (CB23652E)
This circuit for the experimenter of the 60s allowed testing the most common transistors in the...