With this circuit from the September 1989 issue of Radio Electronics magazine, it is possible to receive VLF band signals on a common radio, just like the signals emitted by the previous receiver.
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...