We found this circuit in the Linear Applications Handbook by National Semiconductor. The circuit is from 1994, but it can be made with more modern equivalent components.
The transistor in this circuit can be the BC548 or any general-purpose equivalent, and the phone...
This circuit, from the 1990 Motorola Dataar Databook, can use any LM139 series comparator. The circuit...
This circuit is from a manual of JFETs and can be implemented with the BF245 and for the bipolar...