Found in American documentation from the 90s, this circuit can measure frequencies on a linear scale from 0 to 1 kHz. The components can be changed to measure other ranges.
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....