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.
We found this circuit in a 1967 Electronics World. The circuit can be designed to control other...
The presented circuit monitors a line indicating if it is busy if there is an extension. The...
This circuit is from old documentation. The operational amplifier is uncommon nowadays. The...