This circuit was found in a Radio Electronics from 1970. The coupler with LDR is from that time, but the circuit can be adapted to use modern components and act as a shield.
In the figure we have the conventional configuration of a filter that allows only the frequencies above...
This hysteresis comparator was found in the 1994 National Semiconductor Linear Applications...
The circuit used is the 4093 configured as an inverter in this application from Radio Electronics from...