Found in documentation from the 90s, this circuit triggers a relay with inductive signals applied to a link. The sensitivity depends on the frequency and the power supply depends on the relay used.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This LED signaling circuit uses a programmable unijunction transistor (PUT) as a relaxation...
This is a commercial circuit from a Japanese publication from 1972. The circuit uses a FET that...