With this circuit found in an American documentation from 1969 it is possible to vary the selectivity (Q factor) of a tuned circuit. The operational is from that time.
This circuit was obtained from a Popular Electronics of the 90's. Any power MOSFET can be used as...
This alarm circuit with sensors in series was found in the Electronics Now magazine of June 1994....
This Radio Electronics circuit, in an 80's edition, decodes the 67 kHz signal used at the time to...