This circuit was found in a Popular Electronics from the 90s. The circuit uses a general purpose JFET. The coils depend on the signal range that must be amplified.
Originally designed for WWV (Standard Time Station) signals, it lets 1 kHz tones pass by rejecting...
This circuit is inserted in series with an incandescent lamp causing it to flash at a frequency...
This circuit locks after a short touch on the sensor. The circuit can be powered by voltages from 5...