Found in RCA documentation from the 90s, this circuit uses components from that time. CA3019 is uncommon today and can be replaced by discrete diodes.
With this circuit from Popular Electronics it is possible to determine the characteristics of a microammeter...
This circuit can be implemented with other operational ones. We found it in documentation from the...
I found this circuit in a 1979 documentation. The phone must be at least 1k magnetic or crystal...