Using a 4046 PLL this circuit was obtained from the October 1994 issue of Electronics Now magazine. The circuit can be modified to generate signals at other frequencies.
We found this configuration in a 70s American documentation. The circuit makes use of operational...
Many oscilloscopes have high impedance inputs that make it difficult to observe high frequency signals...
Using a sensitive crystal or ceramic microphone, this circuit can pick up faint sounds through a wall....