By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. This gives a beat note of 250hz to 0 to 250hz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5hz ( motor boating ) any slight change being noticeable.
Any 9v battery PP3 is ideal.
2 off 220uF 16v electrolytic.
5 off .01uF polyester.
5 off .1uF polyester.
All resistors 1/4 watt 5%
6 off 10k
1 off 1K
1 off 2.2m ===== 2.2 Mega ohm
2 off 39k
All BC 184B, or 2N3904, or 2N2222A. Just about any small signal npn with a gain of 250+ will do. There are hundreds to choose from.
A 2.5 inch 8 ohm speaker will work but headphones or earpiece are preferable the higher the impedance the better.
Many of the above parts could be salvaged from a broken transistor radio, or purchased from companies like RS Components, Maplin Electronics, Radio Shack, or Digikey who's adds often appear at the top of this page..
Once the components have been obtained the circuit can be built in a few hours using copper clad stripboard, or if you the facilities make a printed circuit board using the layout below. The original layout as below should print out at about 50mm x 100mm. Coils
This is the only tricky part. The search loop is best wound on to a plywood former. Method 1: Cut three circles from some 3mm plywood, one 15cm diameter and two 16cm diameter. Using wood glue make a sandwich with the 15cm circle in the center. When the glue has set you can wind 10 turns of . 25 mm enameled copper wire around the groove in the edge of the former. Connect this coil when finished to the points marked...