In this 2001 circuit we have the way to trigger an SCR with a phototransistor. The circuit appears elsewhere in this section, obtained from publications from different eras.
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...
In this circuit from the 60s, the power supply is made with 22.5 V, which is the voltage of the...
This circuit is from 1967 General Electric documentation. It uses SUS which are not very common...