Found on an 80's Elektor, this circuit uses common components, being the diode a general purpose germanium. The coils select the frequency range of the indicator that reaches 60 MHz.
This circuit generates 50, 100 and 200 Hz signals from a 3.2768 MHz crystal and has outputs...
This circuit allows frequencies between 1.8 and 30 MHz to pass. Data for the coils is provided next to...
The automatic heating system sensor is a diode placed in a waterproof tube as shown in the figure....