We found this simple protection with zener diode in a 1967 Motorola documentation. The zener diode is 18 V, but the components can be changed for operation with other voltages.
This circuit was found in an old documentation, being able to generate signals of frequencies in...
Found in a Popular Electronics from the 90's, this circuit is used to recognize the frequency of a...
Found in a 1990s American documentation, this circuit provides an output voltage approximately 4 times...