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.
This simple timer is from a 1998 publication. Other operational amplifiers can be used, and the...
This very old circuit, from a publication from the 1970s, uses a germanium transistor and two...
This circuit generates signals for adjustments in FIs of 10.7 MHz. The transistors can be BC548 or...