This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
Another way to increase the sensitivity of the circuit with the amplification of the sensor...
We found this circuit in the Radio Electronics magazine of January 1986. The frequency is given by the...
This circuit was found in the September 1984 issue of Radio Electronics magazine. The circuit is...