This circuit for signals up to 30 MHz can improve your reception in the medium and short-wave band. We found it in the Radio Electronics magazine of February 1989.
This circuit can use a pendulum sensor. We found this configuration in a 1994 publication. The...
This circuit is from a 1964 Electronic Design. The sensor is an LDR and the transistors are...
This circuit works both as a preamplifier and activates a relay when speaking in front of the...