This circuit covers 400 kHz to 40 MHz in three bands according to the selected coil. The double MOSFET allows equivalent, since the original is not easy to obtain.
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...