Found in an Electronics Now from the 90s, this circuit appears elsewhere in this section and website. The diodes are for general use and the basic frequency is given by C.
This frequency shift modulation circuit was found in the November 1994 issue of Electronics Now...
The search coil of this detector consists of 140 turns of wire 18 in an 8 inch diameter form. The...
This circuit was found in American documentation from the 60's. The transistors are made of...